Stoichiometric Calculations

Question 1

Percentage composition formula

Answer 1

. Molar mass of
element
%element = - - - - - - - - - - - - - - - -x 100
Molar mass of
compound

Question 2

Steps to find empirical formula

Answer 2

1.find percentage composition
2. Calculate no. of moles of each element present in 100g (so that you can simply use %)
3. Write down ratio and simplify (multiply to get whole number ratio)
4. Express empirical formula

Question 3

Steps to determine molecular formula

Answer 3

1. Calculate molar mass of empirical formula
2. Divide given molar mass by calculated molar mass
3. Multiply empirical formula by the answer

Question 4

Steps to find limiting reagent

Answer 4

1. Balance equation
2. Convert masses of reactants to moles
3. Write down mole ratio from chemical equation
4. Use the value of the actual no. of moles to determine how much of the other is required according to the equation. (the one that has less than required is the limiting reagent)
5. Using the mole ratio of the equation, you can now determine the amount of product actually made (n=m/M)

Question 5

Steps in calculating percentage yield

Answer 5

1. Determine limiting reagent
2. Calculate theoretical yield from limiting reagent
3. Sub in to formula:
   %yield = (actual yield ÷ theoretical yield) x 100

Question 6

Steps in calculating percentage purity

Answer 6

1. Determine no. of moles of product
2. From balanced equation, determine ratio between reactants and products
3. Using ratio, determine no. of moles of reactant
4. Determine mass of pure reactant
5. Sub into formula:
   %purity= mass of pure substance÷ mass of sample x100