Chemistry 11: Mole and Percentage Composition Practice Problems
These questions are to help you verify that you understand mole conversions and need
to be done for tomorrow. Be careful with your calculations as some of them relate to a
lab we are performing this week. *You are handing this in so be clear on your
explanations and your calculations!
1. The above chemical is known as a triglyceride (a type of fat).
a) How many of each element are present?
b) What is the molecular weight of this element?
c) If you needed to obtain 0.26 mols of this fat, how many grams would you
need to measure?
2. Look up the formula for sucrose.
a) What is the structure?
b) What is the percent composition of each element?
c) If you needed 150g of this substance, how many moles would that equal?
3. Having found the structure of sucrose and solved for the molecular weight, how
many grams would be needed to find 0.49 mols of this substance?
4. Look up sodium bicarbonate.
a) What is the formula for sodium bicarbonate?
b) What is the percent composition by mass of sodium bicarbonate?
c) What is sodium bicarbonate mostly used for? Give some examples.
d) What is the common name for sodium bicarbonate?
e) How many moles are in 5g of sodium bicarbonate?
5. You have an unknown substance that consists of two elements, AA and BB.
We know the percentage composition for unknown BB = 39.4% and for AA = 60.6%
and the molar mass of the substance in total is 58.4 g/mol.
a) What are your unknown AA and BB?
b) What is your substance?
c) What is it more commonly known and used for?
d) If I wanted to obtain 0.086 mols of this substance, how many grams would we
need?
6. Research vanillin.
a) What is the chemical structure of vanillin (look on the internet)?
b) What is the molecular weight of vanillin?
c) If you had a solution that had 3.5 mols for every litre of solution, what would the
concentration be (mols/L)?
d) How many grams of vanillin are in this solution?
e) If you wanted to obtain 5.32g of vanillin for an experiment, how much solution
would you require (you will need to play with your numbers and formulas a bitlook in the text book for help if you need it).
f) What would the difference be between naturally-derived vanillin and synthesized
vanillin (with regards to structure)? This question might require research.
7. You have an unknown liquid in your laboratory with the structure X2O.
a) The molecular mass is about 18. What is X?
b) What are the percentage compositions of each element? Why is one so much
larger than the other?
c) If you required 5 mols of your substance, how many grams would that equal?
d) It can be difficult to express liquids in terms of weight. What is another
measure you can use? What is the conversion factor to go from grams to your
new measurement?
e) Research the density of your liquid to verify that your weight conversion to
the new measurement is correct. Can you use this conversion factor for all
liquids? Why or why not? Provide examples to support your argument.
8. You have an unknown substance that was just recently brought into the lab. They
have told you that the concentration of the compound you are testing in this sample
is 2.7 mol/L. You want to test your sample but you need to place precisely 0.059
mols into your equipment. How much of your substance would you need to measure
out to get the desired mols of your compound?