MCAT Physical : Molecular Weight, Molecular Formula, and Moles

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

Example Question #21 : Stoichiometry And Analytical Chemistry

Manganese forms a number of oxides, one of which is composed of 72% manganese by mass. Which of the following is the formula for this oxide?

Possible Answers:

Correct answer:

Explanation:

Given that the molar mass of oxygen is about 16g, and molar mass of manganese is about 55g,  contains 165g of manganese and 64g of oxygen, for a total of 229g.

165g/229g = 0.72

So, the ratio of manganese to oxygen in this compound is 72% manganese by mass.

Example Question #22 : Molecular Weight, Molecular Formula, And Moles

What is the mass percentage of carbon in glucose (C6H12O6)?

Possible Answers:

96%

25%

40%

It depends on the amount of glucose.

Correct answer:

40%

Explanation:

In order to find the mass percentage of an atom in a molecule, start by finding the total mass of one mole of the molecule. Glucose has 180 grams per mol.

Next, we determine the mass of the carbon atoms in one mole of the molecule. One carbon mole has a mass of 12 grams. Multiplied by the six carbon atoms in glucose gives a mass of 72 grams.

Finally, we divide the mass of carbon by the mass of the molecule.

So, 40% of glucose's mass is made up of carbon.

Example Question #22 : Stoichiometry And Analytical Chemistry

What is the mass percentage of aluminum in aluminum (III) oxide?

Possible Answers:

Correct answer:

Explanation:

Aluminum oxide has the formula .

Aluminum has a molecular weight of , and oxygen has a weight of . Using these values, we can calculate the molecular weight of aluminum oxide.

The mass percentage is given by the mass of aluminum divided by the total molecular weight.

Example Question #21 : Stoichiometry And Analytical Chemistry

What is the percentage by weight of sodium in sodium sulfate?

Possible Answers:

Correct answer:

Explanation:

Sodium sulfate is given by the formula:

To find the percentage by weight, we will need to divide the mass of sodium in the molecule by the total molecular mass.

Convert the ratio to a percentage.

Example Question #651 : Mcat Physical Sciences

Which of the following samples contains a larger mass of hydrogen?

Possible Answers:

The samples contain equal amounts of hydrogen

We must know the density of each compound in order to solve

Correct answer:

Explanation:

Start by calculating the percentages of hydrogen in each compound by using molar mass ratios.



Next, take the percentages of the given sample masses to determine the total mass of hydrogen in each.

We see that there is a larger mass of hydrogen in the hydrogen cyanide sample.  

Example Question #651 : Mcat Physical Sciences

How many grams of nitrogen are in 50g of ammonium sulfate?

Possible Answers:

10.6g

5.9g

5.3g

11.6g

12.3g

Correct answer:

10.6g

Explanation:

First convert grams of ammonium sulfate to moles, then use the mole-to-mole ratio between nitrogen and ammonium sulfate. Finally, convert moles of nitrogen back into grams.

Example Question #21 : Stoichiometry And Analytical Chemistry

What is the molecular weight of NaCl? 

Molar mass of Na = 23g/mol

Molar mass of Cl = 35.5g/mol

Possible Answers:

58.5amu

58.5g/amu

58.5g/mol

58.5amu/mol

Correct answer:

58.5amu

Explanation:

To find the molecular weight (mass) of a molecule, simply add up the atomic weights of each atom within the molecule. The units used will be amu (atomic mass units) for molecular weight and g/mol for molar mass.

Example Question #21 : Stoichiometry And Analytical Chemistry

Convert 23g of water to moles.

Possible Answers:

0.8mol

2.6mol

1.4mol

1.3mol

Correct answer:

1.3mol

Explanation:

First find the molar mass of water (H2O). You should be comfortable with the molar masses of hydrigen and oxygen from memory to reduce time on the MCAT exam.

Next, solve for moles.

Example Question #655 : Mcat Physical Sciences

Calculate the mass percent of sulfur in sulfuric acid .

Possible Answers:

Correct answer:

Explanation:

To calculate the mass percent, calculate the the individual masses of each element. Then divide the mass of sulfur by the total mass of the molecule. Since there is only one sulfur atom in sulfuric acid, the mass of sulfur in one atom is .

Example Question #652 : Mcat Physical Sciences

Compounds can be distinguished from each other by using their molecular weights. The molecular weight of a compound depends on the individual atomic weights of the elements and the amount of each element present in the compound. Consider hexane for example. Hexane has a molecular formula of . This means that it has 6 carbon atoms and 14 hydrogen atoms. To calculate the molecular weight of hexane, we can simply look up the molecular weight of carbon and hydrogen from the periodic table, multiply each molecular weight by the number of atoms (6 for carbon and 14 for hydrogen), and sum the two numbers. The molecular weight of an element is always given in . One mole is the defined as the number of atoms in twelve grams of carbon-12.

A researcher is trying to determine the molecular formula of a hydrocarbon molecule. He measures the molecular weight to be . He also observes that the molecule has one  bond. What is the ratio of the number of carbon to hydrogen atoms in this molecule?

Possible Answers:

Correct answer:

Explanation:

The empirical formula for hydrocarbon is , where  is the number of carbons. This is only true if the hydrocarbon has no  bonds or ring structures. The question states that there is one  bond; therefore, the hydrocarbon will lose two hydrogen atoms (). Note that for every  bond and every ring the hydrocarbon is associated with the loss of two hydrogen atoms. If we calculate the molecular weight of hydrocarbons with different  values () we will find that the nine-carbon hydrocarbon (with  bond), nonene, has a molecular weight of . The molecular formula for nonene is ; therefore, the ratio of carbons to hydrogens is 9:18 or 1:2.

You can also calculate the ratio by simply looking at the empirical formula of this molecule (). There will be twice as many hydrogen atoms as carbon atoms; therefore, ratio of carbon to hydrogen will be 1:2.

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