MCAT Physical : Molecular Weight, Molecular Formula, and Moles

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

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Example Question #657 : 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.

How many atoms of oxygen are found in 1g of oxygen gas?

Possible Answers:

Correct answer:

Explanation:

To solve this question, we need to first convert grams to moles, then moles to atoms. The molecular weight (MW) of oxygen gas,  is

The amount of moles in 1g of oxygen gas is

There are  atoms in  (this is called the Avogadro’s number); therefore, the number of atoms in  of  is

Example Question #658 : 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.

Consider two carbohydrates A and B. Molecule A is a 6-carbon carbohydrate and has twice as much molecular weight as molecule B. What can you conclude about molecule B?

Possible Answers:

Oxygen contributes to about  of the molecular weight of both molecules A and B

None of these

Molecule A has twice as many oxygen atoms as carbon atoms

Molecule B has twice as many oxygen atoms as molecule A

Correct answer:

Oxygen contributes to about  of the molecular weight of both molecules A and B

Explanation:

The empirical formula for carbohydrates is , where  is the number of carbon atoms. The question states that molecule A has 6 carbons; therefore,  and the molecular formula for molecule A is . The molecular weight (MW) of this compound is calculated using the molecular weight of each atom. The MW of each atom is obtained from the periodic table and is multiplied by the number atoms.

So, the molecular weight of molecule A is . Since it is also a carbohydrate, molecule B will have the same empirical formula as molecule A. Molecule B has half the molecular weight of molecule A; therefore, molecule B must have half the atoms as molecule A. The molecular formula of molecule B is . Note that MW of  is .

The MW contribution of oxygen to each compound is calculated as follows.

%MW of oxygen for molecule A = 

%MW of oxygen for molecule B =  

Therefore, oxygen contributes to 53% of MW in both compounds.

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