C3 (quantitative chemistry)

Question 1

what is the law of conservation of mass?

Answer 1

the mass of the products in a chemical reaction must equal the mass of the reactants (what goes in, must come out).

Question 2

how do you find the relative formula mass of a compound?

Answer 2

add together all of the relative atomic masses of all the atoms in the compound’s formula.

Question 3

how do you find the percentage mass of an element?

Answer 3

1. find the relative atomic mass of the element
2. multiply this by how many there are of the element in the equation
3. divide this number by the relative formula mass of the entire compound
4. multiply this last number by 100 to turn it into a percentage

Question 4

what is the an exception to the law of conservation of mass?

Answer 4

when reactions involve a gas, the mass may seem to change, as gas can come from the air, or float off into the air.
- completing reactions in a sealed container
means that the reaction will balance
perfectly.

Question 5

how is relative atomic mass and a mole linked together?

Answer 5

the relative atomic mass of carbon is 12.
in 12g of carbon, there is 1 mole of carbon atoms.

when we take the relative atomic mass of any element in grams, there’s one mole of atoms in that element.

Question 6

describe moles:

Answer 6

• chemical amounts are measured in moles. the symbol is ‘mol’.
• 1 mole of a substance is equal to the amount of that substance that contains 6.02 x 10^23 particles (atoms, molecules, ions, electrons - depends on which substance we’re talking about).

Question 7

explain avogadro’s constant:

Answer 7

• the mass of 6.02 x 10^23 particles of any substance will be exactly the same number as that substance’s relative formula mass in grams.

e.g. carbon’s relative atomic mass is 12. therefore one mole of oxygen is equal to 12g.

Question 8

how do you calculate the number of moles in an element?

Answer 8

1. find the mass of the target element (it’s usually given to you in the question)
2. divide the mass by the relative atomic mass of the element

Question 9

how do you use moles to balance equations?

Answer 9

1. calculate the number of moles in all of the chemicals in the reaction
2. place the number of moles in front of every term in the equation (e.g. there is 1 mole in hydrogen, so put it as 1H2)
3. divide all the terms by the lowest number of moles, to ensure that the ratio is as low as possible

Question 10

what is a limiting reactant, compared to the excess reactant?

Answer 10

• it’s common to use an excess of one of the reactants to ensure that all of the other reactant it used. this reactant that is all used up is the limiting reactant as it limits the amount of product.

Question 11

what is meant by concentration, and what is its unit?

Answer 11

the mass of a solute in a given volume of solution. it’s measured in g/dm^3

Question 12

how would you calculate the concentration of a solution?

Answer 12

1. divide the mass of the chemical (in grams) by the volume of the solution (in dm^3)

Question 13

how do the mass and volume of a solute affect the concentration?

Answer 13

• increasing the mass of the solute, but keeping the volume the same, increases the concentration
• increasing the volume of the solute, but keeping the mass the same, decreases the concentration

Question 14

what is the yield of a reaction, and what is it measured in?

Answer 14

the amount of desired product that you get. it can be measured in grams or moles.

Question 15

why might the actual yield differ from the theoretical yield?

Answer 15

• the reactants might not all react (e.g. if the reaction hasn’t been given sufficient time, or if it’s a reversible reaction)
• there could be side reactions (where the reactants react to produce a different product than expected)
• some of the product could be lost during the process (e.g. gaseous products could float off into the air)

Question 16

how do you calculate the percentage yield?

Answer 16

1. take the actual yield and divide it by the theoretical yield
2. multiply it all by 100 to give us a percentage

Question 17

what is meant by atom economy?

Answer 17

a measure of how much of the reactants go into forming the product that you actually want.

Question 18

how do you calculate the atom economy of an reaction?

Answer 18

1. find the relative atomic mass of your desired product (s)
2. divide this number by the relative formula mass of all of the reactants
3. multiply this number by 100 to turn it into a decimal

Question 19

why is it important to consider atom economy?

Answer 19

1. raw materials are expensive (if they’re being converted into waste products, the process will be less profitable)
2. it’s less sustainable to use large quantities of reactants to make tiny quantities of products
3. waste products themselves can be expensive to get rid of, and can damage the environment

Question 20

what are the best solutions for a low atom economy?

Answer 20

1. use a more efficient reaction
2. find a use for the waste products, so that they’re useful by-products instead

Question 21

how would you solve a titration calculation question?

Answer 21

1. begin by writing out the letters: ‘c=’, v=’ and ‘n=’ under both the acid and the alkali in the equation
2. enter the information we’ve been given (ensure to convert any volumes into dm^3, by dividing by 1000)
3. you will now have enough information to calculate the number of moles in one of the solutions (n=c x v)
4. this number will be the same number of moles in the other solution (or you’ll have to do a simple multiplication/division to get the number of moles in the other solution)
5. calculate the unknown term in the second solution, by rearranging and using the n = c x v equation - THIS WILL GET YOU THE CONCENTRATION IN MOL/DM^3
6. to get the concentration in g/dm^3, multiply the mol/dm^3 number by the relative atomic mass of the solution

Question 22

how do you find the volume of a gas?

Answer 22

1. find the number of moles in the gas
2. multiply this number by the molar gas volume (24)

Question 23

what are the only conditions for the molar gas volume to be correct?

Answer 23

the molar gas volume only works for gases at room temperature and pressure. changing the temperature/pressure will change the molar gas volume.
- however, in exams, molar gas volume of 24 is the only one you’ll need to use