Molecules and stoichiometry are two critical topics that are tested on the MCAT chemistry section.
Molecules are the building blocks of everything around us, and understanding their properties and behavior is crucial for success on the MCAT. This includes understanding the molecular structure, chemical bonding, intermolecular forces, and the properties of solutions. These high-yield MCAT topics are not only essential for understanding the behavior of molecules but also the reactions they undergo.
Stoichiometry is another critical concept tested in the MCAT chemistry section. It is the study of the quantitative relationships between reactants and products in a chemical reaction. Stoichiometry is essential because it allows you to determine the exact amount of reactants needed to produce a desired product or the amount of product that can be produced from a given amount of reactants. It also allows you to calculate important reaction parameters such as percent yield and limiting reagents.
In the MCAT chemistry section, you can expect to see questions that require you to apply your knowledge of molecules and stoichiometry to analyze and solve complex problems. You may be asked to determine the molecular structure of a given compound, identify intermolecular forces, or calculate the concentration of a solution. Additionally, you may be asked to balance MCAT chemistry equations, calculate reaction stoichiometry, or determine the percent yield of a reaction. Luckily, you are allowed to use the Periodic Table during the MCAT.
In this article, we’ll provide all of the content you need to know to ace chemistry questions in particular with MCAT concepts about molecules and stoichiometry. In addition, we will provide practice questions with answers for you to test your knowledge with.
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Moles and molecular mass
The concept of moles and molecular mass is fundamental to understanding chemistry. In the context of the MCAT, these concepts are essential for success in the Chemical and Physical Foundations of Biological Systems section. In this section, we will provide an introduction to moles and molecular mass, including definitions, calculations, and examples.
What is a mole?
A mole is a unit of measurement that is used to express the amount of a substance in a given sample. One mole of a substance is defined as the amount of that substance that contains as many elementary entities (such as atoms, molecules, or ions) as there are atoms in 12 grams of carbon-12.
The number of elementary entities in one mole of a substance is known as Avogadro's number, which is approximately 6.02 x 10^23. This means that one mole of carbon-12 contains 6.02 x 10^23 carbon atoms, while one mole of water contains 6.02 x 10^23 water molecules.
What is molecular mass?
Molecular mass, also known as molecular weight, is a term used in chemistry to describe the mass of a molecule. It is calculated by summing the atomic masses of the atoms in the molecule. Molecular mass is expressed in atomic mass units (amu) or grams per mole (g/mol).
To calculate the molecular mass of a compound, we need to know the atomic masses of the elements present in the compound. These atomic masses can be found on the periodic table which we are allowed to use in MCAT.
For example, the molecular mass of water (H2O) is calculated by adding the atomic masses of two hydrogen atoms and one oxygen atom. The atomic mass of hydrogen is 1.008 amu, while the atomic mass of oxygen is 15.999 amu. Therefore, the molecular mass of water is 2(1.008 amu) + 15.999 amu = 18.015 amu, or 18.015 g/mol.
Example problems
Here are some examples of MCAT problems that involve the concept of moles and molecular mass:
What is the mass, in grams, of 2.5 moles of sodium chloride (NaCl)?
How many moles of carbon dioxide (CO2) is produced when 5.0 moles of oxygen (O2) react with excess methane (CH4) in the following reaction:
CH4 + 2O2 → CO2 + 2H2O
What is the molarity of a solution containing 3.0 moles of sodium chloride (NaCl) in 500 mL of solution?
What is the molecular mass of glucose (C6H12O6)?
What is the mass of 0.5 moles of methane (CH4)?
Molecules
In the context of the MCAT, understanding the importance of molecules is critical because it is a fundamental concept in many areas of biology and chemistry, including biochemistry and organic chemistry, which are both heavily tested on the exam.
A molecule is a group of two or more atoms held together by chemical bonds. These atoms can be of the same element or different elements. For example, a molecule of oxygen (O2) contains two atoms of oxygen bonded together, while a molecule of water (H2O) contains two atoms of hydrogen and one atom of oxygen bonded together.
In this section, we will discuss the representation of molecular formulas and types of chemical reactions.
Representations of molecular formulas
Molecular formulas are representations of chemical compounds that show the types and numbers of atoms that make up the compound. These formulas are written using chemical symbols and numbers to indicate the composition of the molecule.
The chemical symbol represents the element, and the subscript number indicates the number of atoms of that element in the molecule. For example, the molecular formula for water is H2O, which indicates that there are two atoms of hydrogen and one atom of oxygen in each molecule of water.
Here are the different representations of molecular formulas and examples for each one:
Chemical Formula: This is the simplest way to represent a molecular formula and it just lists the type and number of atoms in the molecule. For example, the chemical formula for water is H2O, which means it contains two hydrogen atoms and one oxygen atom. Other examples include:
Carbon Dioxide: CO2
Ethanol: C2H5OH
Glucose: C6H12O6
Methane: CH4
Lewis Structure: This structure uses lines and dots to represent the bonds and electrons in a molecule. For example, the Lewis structure of carbon dioxide (CO2) shows two double bonds between the carbon atom and the oxygen atoms. This structure uses lines and dots to represent the bonds and electrons in a molecule. Other examples include:
Water: H-O-H
Ammonia: H-N-H
Methanol: H-O-C-H
Ethene: H2C=CH2
Ball-and-stick model: This model uses balls to represent atoms and sticks to represent the bonds between atoms.
Space-filling model: This model uses spheres to represent atoms, and the size of the spheres is proportional to the size of the atoms.
Condensed structural formula: This formula shows the molecular formula, but with certain bonds and atoms condensed or omitted.
It is important to note that different representations can be used to depict the same molecule, and each representation provides a different level of detail about the molecule's structure and properties.
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Types of chemical reaction
You must have a firm grasp of the fundamental concepts of chemistry, including the different types of chemical reactions. This knowledge is crucial in understanding chemical reactions that take place within biological systems, including the human body
There are six main types of chemical reactions that you should be familiar with: combination or synthesis, decomposition, single displacement or substitution, double displacement or metathesis, acid-base or neutralization, and redox or oxidation-reduction reactions.
Note that some reactions can fall under multiple categories, depending on the perspective taken. For instance, combustion reactions can be viewed as redox or as a subtype of combination reactions.
Example Problems
1. Which of the following molecules has the same molecular formula as ethylene glycol, C2H6O2?
A) Dimethyl ether, CH3OCH3
B) Acetaldehyde, CH3CHO
C) Acetic acid, CH3COOH
D) Methyl acetate, CH3COOCH3
Explanation:
To solve this problem, we need to compare the molecular formula of ethylene glycol, C2H6O2, with the molecular formulas of the given options. We can do this by counting the number of atoms of each element in the formula.
The molecular formula of dimethyl ether is CH3OCH3, which has 4 carbon atoms, 10 hydrogen atoms, and 1 oxygen atom. This is not the same molecular formula as ethylene glycol.
The molecular formula of acetaldehyde is CH3CHO, which has 3 carbon atoms, 6 hydrogen atoms, and 1 oxygen atom. This is not the same molecular formula as ethylene glycol either.
The molecular formula of acetic acid is CH3COOH, which has 2 carbon atoms, 4 hydrogen atoms, and 2 oxygen atoms. This is not the same molecular formula as ethylene glycol.
The molecular formula of methyl acetate is CH3COOCH3, which has 3 carbon atoms, 6 hydrogen atoms, and 2 oxygen atoms. This is the same molecular formula as ethylene glycol.
Therefore, the correct answer is D) Methyl acetate, CH3COOCH3.
2. What type of chemical reaction is represented by the following equation?
2H2O2 (aq) → 2H2O(l) + O2 (g)
A) Acid-base reaction
B) Combustion reaction
C) Decomposition reaction
D) Single replacement reaction
Explanation:
In a decomposition reaction, a single reactant breaks down into two or more simpler products. In the given equation, the reactant hydrogen peroxide (H2O2) breaks down into water (H2O) and oxygen gas (O2).
An acid-base reaction involves the transfer of a proton (H+ ion) between two substances. This reaction does not involve any proton transfer and hence is not an acid-base reaction.
A combustion reaction is a type of reaction in which a substance reacts with oxygen to produce heat and light. While oxygen is a reactant in this equation, the other reactant (hydrogen peroxide) does not burn in the presence of oxygen to produce heat and light. Therefore, it is not a combustion reaction.
A single replacement reaction involves an element replacing another element in a compound. There are no elements present in this equation and hence it is not a single replacement reaction.
Therefore, the correct answer is C) Decomposition reaction.
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Stoichiometry
Stoichiometry is a fundamental concept in chemistry that involves the calculation of the quantities of reactants and products involved in a chemical reaction. It is essential for understanding the relationship between the amounts of reactants and products in a reaction, as well as for predicting the outcomes of chemical reactions. In this section, we will discuss balancing chemical equations and finding the theoretical yield.
A step-by-step guide for balancing equations
In stoichiometry, we use the balanced chemical equation to determine the molar ratios of reactants and products involved in a reaction. A balanced chemical equation shows the mole ratios of reactants and products and indicates the number of moles of each reactant and product that participate in the reaction.
Balancing chemical equations involves adjusting the coefficients (numbers in front of the chemical formulas) of the reactants and products in the equation so that the number of atoms of each element is equal on both sides of the equation. The following steps can be followed to balance a chemical equation:
Example: Balancing the equation for the combustion of propane (C3H8) with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O)
Step 1: Write the unbalanced equation
C3H8 + O2 → CO2 + H2O
Step 2: Count the number of atoms of each element on both sides of the equation
Reactants: 3 C, 8 H, 2 O
Products: 1 C, 2 H, 3 O
Step 3: Balance the equation by adding coefficients to the reactants and products
C3H8 + 5O2 → 3CO2 + 4H2O
Step 4: Verify that the number of atoms of each element is the same on both sides of the equation
Reactants: 3 C, 8 H, 10 O
Products: 3 C, 8 H, 10 O
Now that the equation is balanced, we can use stoichiometry to calculate the amount of reactants and products involved in the reaction. For example, if we wanted to know how many moles of oxygen are needed to react with 2 moles of propane, we would use the balanced equation to set up a ratio:
Step 5: Use stoichiometry to calculate the amount of reactants and products involved in the reaction
C3H8 + 5O2 → 3CO2 + 4H2O
2 moles of C3H8 + x moles of O2 → y moles of CO2 + z moles of H2O
From the balanced equation, we can see that 1 mole of C3H8 reacts with 5 moles of O2, so 2 moles of C3H8 will react with:
2 moles C3H8 x (5 moles O2/1 mole C3H8) = 10 moles of O2
Therefore, 10 moles of oxygen are needed to react with 2 moles of propane to produce the products carbon dioxide and water.
In conclusion, balancing chemical equations is a crucial step in stoichiometry that ensures the law of conservation of mass is obeyed. By following a few simple steps, we can balance equations and use stoichiometry to make quantitative predictions about chemical reactions.
Determining theoretical yield
Determining theoretical yield is a concept commonly discussed in chemistry, specifically in stoichiometry. The theoretical yield is the maximum amount of product that can be obtained from a given amount of reactants, assuming complete conversion of all reactants and no losses during the reaction. It is determined by calculating the amount of product that would be produced if the reaction proceeded perfectly according to the balanced chemical equation.
To calculate the theoretical yield, one must first balance the chemical equation for the reaction of interest. Then, the stoichiometry of the balanced equation is used to convert the given amount of reactant to the amount of product that would be produced. The result is the theoretical yield.
For example, let's consider the reaction between hydrogen gas (H2) and oxygen gas (O2) to produce water (H2O). The balanced chemical equation for this reaction is:
2H2 + O2 → 2H2O
If we start with 10 grams of hydrogen gas, we can calculate the theoretical yield of water that could be produced by using the stoichiometry of the balanced equation. According to the equation, 2 moles of hydrogen gas react with 1 mole of oxygen gas to produce 2 moles of water. Therefore, 10 grams of hydrogen gas is equivalent to 0.556 moles of H2. Since the ratio of H2 to H2O is 2:2, we know that the theoretical yield of water would be 0.556 moles. To convert this to grams, we can use the molar mass of water (18.015 g/mol) and multiply by the number of moles:
Theoretical yield of water = 0.556 mol H2O × 18.015 g/mol H2O = 9.99 g H2O
Therefore, the theoretical yield of water that could be produced from 10 grams of hydrogen gas is 9.99 grams.
Example Problems
1. What is the correctly balanced chemical equation for the reaction between aluminum (Al) and sulfuric acid (H2SO4) to produce aluminum sulfate (Al2(SO4)3) and hydrogen gas (H2)?
2. What is the balanced chemical equation for the reaction between sodium hydroxide (NaOH) and hydrochloric acid (HCl)?
3. What is the theoretical yield of silver chloride (AgCl) that can be produced from 5.00 g of silver nitrate (AgNO3) reacting with excess hydrochloric acid (HCl)?
AgNO3(aq) + HCl(aq) → AgCl(s) + HNO3(aq)
MCAT test prep tips and strategies
Understand the basics
Before attempting any MCAT biology practice questions related to molecules and stoichiometry, make sure you have a solid understanding of the basic concepts. Review the properties of atoms, molecules, and chemical bonds, as well as the principles of stoichiometry and balancing chemical equations.
Read carefully
For passage-based questions, read the entire passage carefully before attempting to answer any questions. Pay attention to any graphs, tables, or figures, and make note of any key concepts or details that are mentioned.
Identify the question type
For standalone questions, identify the type of question being asked, such as multiple-choice or fill-in-the-blank. This can help you focus your attention on the relevant information and determine the correct answer.
Use the process of elimination
For multiple-choice questions, use the process of elimination to narrow down your choices. Eliminate any answers that are clearly incorrect, and then focus on the remaining options.
Apply your knowledge
Use your knowledge of molecules and stoichiometry to answer the question. Make sure you understand the relationships between the variables involved and apply any relevant formulas or equations.
Practice
Practice is key to improving your skills and preparing for the MCAT. Use sample questions and practice exams to build your confidence and familiarize yourself with the types of questions you may encounter on the exam.
By following these tips and strategies, you can improve your performance on MCAT questions related to molecules and stoichiometry, and feel more confident on exam day.
Practice questions and answers
Here're some more questions to test your MCAT knowledge!
1. A chemist needs to prepare 500 mL of a 0.5 M solution of sodium chloride (NaCl). How many grams of NaCl should be dissolved in water to prepare this solution?
A. 11.69 g
B. 29.23 g
C. 58.46 g
D. 116.92 g
Answer:
The molar mass of NaCl is 58.44 g/mol. To prepare 0.5 moles of NaCl in a 500 mL solution, we need:
0.5 mol/L x 0.5 L = 0.25 moles of NaCl
The mass of NaCl needed is:
0.25 mol x 58.44 g/mol = 14.61 g
Therefore, the correct answer is (B) 29.23 g.
2. How many moles of nitrogen gas (N2) are produced when 10 moles of ammonia (NH3) react completely with excess oxygen gas (O2) according to the balanced chemical equation: 4NH3 + 5O2 -> 4NO + 6H2O
A. 6.25 moles
B. 8.00 moles
C. 10.00 moles
D. 12.50 moles
Answer:
According to the balanced chemical equation, 4 moles of ammonia react with 5 moles of oxygen to produce 4 moles of nitrogen gas. We can use this ratio to find the number of moles of nitrogen gas produced from 10 moles of ammonia:
10 mol NH3 x (4 mol N2 / 4 mol NH3) = 10 mol N2
Therefore, the correct answer is (C) 10.00 moles.
3. What is the molecular formula of a compound that contains 42.11% carbon, 6.28% hydrogen, and 51.61% oxygen by mass, with a molar mass of 180 g/mol?
A. C4H4O4
B. C4H6O4
C. C5H6O5
D. C6H8O6
Answer:
First, we need to convert the mass percentages to mole ratios. Assume we have 100 g of the compound:
42.11 g carbon / 12.01 g/mol = 3.50 moles carbon
6.28 g hydrogen / 1.01 g/mol = 6.22 moles hydrogen
51.61 g oxygen / 16.00 g/mol = 3.23 moles oxygen
Next, we need to determine the simplest whole number mole ratio of the elements. Divide each mole quantity by the smallest value (in this case, 3.23 moles oxygen) and round to the nearest whole number:
Carbon: 3.50 / 3.23 = 1.08, rounded to 1
Hydrogen: 6.22 / 3.23 = 1.92, rounded to 2
Oxygen: 3.23 / 3.23 = 1
The molecular formula is, therefore, C2H4O2, which has a molar mass of 60 g/mol. To determine the multiple of the molecular formula, divide the given molar mass by the calculated molar mass:
180 g/mol / 60 g/mol = 3
The final answer is C6H12O6, which is option (D).
4. What volume of 0.10 M hydrochloric acid (HCl) is required to completely react with 25 mL of 0.20 M sodium hydroxide (NaOH)?
A. 6.25 mL
B. 12.50 mL
C. 25.00 mL
D. 50.00 mL
Answer:
The balanced chemical equation for the reaction between HCl and NaOH is:
HCl + NaOH → NaCl + H2O
To determine the volume of HCl required, we need to use the balanced equation to calculate the number of moles of NaOH and HCl that will react. The stoichiometry of the balanced equation tells us that 1 mole of HCl reacts with 1 mole of NaOH.
n(NaOH) = M × V = 0.20 mol/L × 25 × 10^-3 L = 0.005 mol NaOH
Therefore, 0.005 mol HCl is required to react with all the NaOH. We can use the molarity and volume of HCl to calculate the number of moles of HCl present:
n(HCl) = M × V
0.10 mol/L × V = 0.005 mol
V = 0.005 mol / 0.10 mol/L = 0.050 L = 50.00 mL
The answer is (D) 50.00 mL.
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