Stoichiometry and Basic Chemistry Math

Question 1

Molecular formula

Answer 1

Specify the type and number of each atom in a molecule

Question 2

Empirical formula

Answer 2

Ratio of atoms in a compound

Question 3

Give an example of a a molecular formula and it’s empirical formula

Answer 3

Glucose:
molecular- C6H12O6
empirical- CH2O

Question 4

What is the empirical formula for a 150g sample of 60g carbon, 10g hydrogen, and 80g oxygen?

Answer 4

10g C= 5 mol
10g H= 10 mol
80g O= 5 mol
C5H10O5 –> CH2O

Question 5

Given the empirical formula CH2O, what is the molecular formula if the molar mass of the sample is 180g/mol?

Answer 5

C= 12 g/mol
H= 1 g X 2= 2 g/mol 
O= 16 g/mol
Total mass of CH2O= 30g 
180 g/30g= 6
so, C6H12O6

Question 6

What is the amount in grams of each individual element of a 150 g compound that contains 40% carbon, 7% hydrogen, and 53% oxygen?

Answer 6

Carbon- 150g X 0.4 = 60 g
Hydrogen- 150g X 0.07= 10g
Oxygen- 150g X 0.53= 80g
then convert to moles
On MCAT there is no calculator, so using 100g instead of 150g works too and can see the 1:2:1 ratio

Question 7

What is the percent composition by mass of oxygen in a molecule with the empirical formula CH2O?

Answer 7

Carbon- 1 X 12g= 12 g/mole
Hydrogen- 2 X 1g= 2 g/mol
Oxygen= 1 X 16g= 16 g/mol
Added together = 30 g/mol
C- 12/30= 40%
H- 2/30= 7%
O- 16/30= 50%

Question 8

Identifying limiting reagents

Answer 8

The reagent that makes fewer

Question 9

Percent yield

Answer 9

(Amount of product formed)/
(theoretical yield) X 100
Actual yield: given
theoretical yield: calculated