Chapter 11 - Stoichiometry

Question 1

How do you calculate the % composition of a certain element in a compound?

Answer 1

1. Find the mass number of the element
2. Find the molecular mass of the molecule
3. Divide the mass of the element by the mass of the molecule and multiply by 100

Question 2

Find the % of carbon in CO₂ (don’t need the exact answer just know how to do it)

Answer 2

○mass number of C → 12
○molecular mass of CO₂ → 12+16+16=44
○12/44 x 100 = 27.27%

Question 3

Find the % of nitrogen in (NH₄)₂SO₄ (don’t need the exact answer just know how to do it)

Answer 3

○mass of nitrogen → 14(2) = 28
○molecular mass of (NH₄)₂SO₄ → 14(2)+8(1)+32+16(4)=132
○28/132 x 100 = 21.21%

Question 4

Molecular formula

Answer 4

A formula which shows the number and type of each atom present in a molecule of that compound

Question 5

Empirical formula

Answer 5

A formula showing the simplest whole number ratio of numbers of different atoms present in a molecule

Question 6

if the molecular formula of glucose is C₆H₁₂O₆, what is its empirical formula?

Answer 6

CH₂O

Question 7

How do you find the empirical formula of a compound using % composition?

Answer 7

Divide the % composition of each element by its atomic mass (e.g if there’s 40% carbon, 40 ÷ 12 = 3.333. Then find the simplest ratio of these elements against each other

Question 8

Gravimetric analysis

Answer 8

Analysis of a compound based on masses that it reacts with or is produced

Question 9

How do you calculate empirical formula when given the masses of the reactants and products?

Answer 9

1. Write a word equation of the reaction
2. Work out the mass of the missing reactant
3. Calculate the number of moles of each reactant
4. Find the ratio in which they react

Question 10

How do you calculate the mass of a substance from a balanced equation using the mole concept?

Answer 10

1. Turn the question into moles
2. Relate it back to the balanced equation to find the ratios, and thus the number of moles of reactant or product asked for
3. Turn from moles to mass, volume or particles if asked