C3 - Quantative Chemistry

Question 1

What is the relative formula mass?

Answer 1

All the relative atomic masses of all the atoms in the molecular formula added together.

Question 2

How do you calculate the percentage mass of an element in a compound?

Answer 2

Ar X nunber of atoms of that element
———————————————————
Mr of the compound

Question 3

What is a mole?

Answer 3

An amount of a substance

Question 4

How many is a mole?

Answer 4

Same as Avagadro’s Constant: 6.02 X 10^23

Question 5

What is the formula to find the number of moles in a given mass?

Answer 5

Mass / Mr

Question 6

What is the conservation of mass?

Answer 6

Both the reactants and product have the same mass.

Question 7

How can you balance equations with reacting masses?

Answer 7

1) Divide the mass of each substance by its relative formula mass to find the number of moles.
2) Divide the number of moles of each substance by the smallest number of moles in the reaction.
3) If any numbers aren’t whole numbers, multiply every number by the same amount until they’re all whole numbers.

Question 8

Why are substances added in excess?

Answer 8

To make sure that the other reactant is used up.

Question 9

What is the limiting reactant?

Answer 9

The reactant that is used up in a reaction.

Question 10

What is the relationship between the limiting reactant and the amount of product formed?

Answer 10

Directly proportional.

Question 11

How can you calculate the mass of a product formed in reaction using the limiting reactant?

Answer 11

1) Write out the balanced equation.
2) Work out the Mr of the reactant and product you want.
3) Find out how many moles there are of the substance you know the mass of.
4) Use the balanced equation to work out the molar ratio.
5) Use the molar ratio to calculate the mass.

Question 12

What is the equation to work out the volume of a gas?

Answer 12

Mass of gas
————— X 24
Mr of gas

Question 13

What is the equation to calculate concentration?

Answer 13

Mass Of Solute
————————
Volume Of Solvent

Question 14

How do you convert mol/dm^3 into g/dm^3?

Answer 14

Use the equation mass = moles X Mr

Question 15

What is the equation to calculate the atom economy of a reaction?

Answer 15

Relative formula mass of desired products
———————————————————
Relative formula mass of all reactants

                        X 100

Question 16

What reaction would have the highest atom economy?

Answer 16

A reaction with only one product. The more products there are, the lower the atom economy is likely to be.

Question 17

What is the best way to solve the issue of other products being formed in a reaction?

Answer 17

Find a use for the waste product instead of disposing of it. Most products have more than one reaction, so it is best to use a reaction that gives useful by-products rather than useless ones.

Question 18

What is the equation to calculate percentage yield?

Answer 18

Mass Of Product Actually Made
——————————————— X 100
Maximum Theoretical Mass Of Product

Question 19

What are three reasons why percentage yields are most likely less than 100%?

Answer 19

1) Not all the reactants react to make a product, such as in reversible reactions where products turn back into reactants too.
2) There could be side reactions such as gases in the air reacting with the reactants or impurities within the reaction mixture. Both end up forming extra products other than the ones that are desired.
3) You lose some product when you separate it from the reaction mixture.