Quantitative Chemistry

Question 1

difference between multipliers in normal script vs subscript

Answer 1

a balancing number, written in normal script, multiplies all the atoms in the substance next to it whereas in subscript, just multiplier the atom before it

Question 2

explain observed changes of mass during chemical reactions in non-enclosed systems using the particle model when given a balanced symbol equation

Answer 2

if a gas escapes, the total mass will look as if it has decreased. If a gas is gained, the total mass will look as if it has increased. However, the total mass stays the same if the mass of the gas is included.

Question 3

why is there uncertainty of results whenever a measurement is made?

Answer 3

All measurements have a degree of uncertainty regardless of precision and accuracy. This is caused by two factors, the limitation of the measuring instrument (systematic error) and the skill of the experimenter making the measurements (random error)

Question 4

what is a mole?

Answer 4

One mole, 1 mol, of a substance is the Avogadro constant number (6.02 × 1023) of particles (atoms, molecules, ions or formulae) of that substance

Question 5

how would you calculate a number of particles given the amount of moles and number of particles

Answer 5

avogadro’s constant x amount of moles x number of particles

Question 6

how would you use the RFM of a substance to calculate the number of moles in a given mass of the substance?

Answer 6

mass= RFM x moles
or
amount of atoms (moles)= mass/ RFM

Question 7

how would you calculate the masses of reactants or products when given a balanced symbol equation? (given mass of one substance)

Answer 7

1. draw out table with unknown and known substance at the top
2. in order: mass, RFM, moles, ratio written down the side

Question 8

how would you use moles to write balanced equations when given the masses of the reactants and products

Answer 8

1. work out the number of moles of each substance using mass/ RFM
2. divide by smallest number of moles
3. write these numbers infront of the substances

Question 9

how would you identify/ work out a limiting reactant question?

Answer 9

1. question will usually have an equation with one mole of each substance, so if it asks how much of a product would be made with a number of moles less than 1 of a reactant, it is a limiting reactant question
2. if the numbers are all 1, multiple the mass of the limiting reactant with the RFM of the product

Question 10

how to calculate the mass of a solute in a given volume of solution of known concentration in terms of mass per given volume of solution

Answer 10

Amount of solute in mol or g= concentration of solution in mol or g/dm3 × volume of solution in dm3
* to convert from mol/dm3 to g/dm3, multiply by the relative formula mass
* to convert from g/dm3 to mol/dm3, divide by the relative formula mass

Question 11

explain how the mass of a solute and the volume of a solution is related to the concentration of the solution

Answer 11

• the higher the mass of the solute, the higher the concentration, for a given volume
• so the lower the volume, the higher the concentration

Question 12

explain why it’s not always possible to obtain the calculated or expected amount of a product

Answer 12

• the reaction not going to completion, because it is reversible
• some of the product may be lost when it is separated from the reaction mixture by
  filtering, for example
• some of the reactants may react in ways different to the expected reaction

Question 13

formula for percentage yield of a product

Answer 13

• (mass of product actually made/ maximum theoretical mass of product) x100

Question 14

how to calculate theoretical yield

Answer 14

1. Write down the balanced chemical equation
2. Calculate the relative formula masses of the substances in the question
3. Use the balanced chemical equation to work out the reacting masses based on the relative formula masses (how much product makes how much reactant)
4. Work out the reacting masses for 1 g (or 1 kg or 1 tonne if different mass units are used)
5. Scale up the reacting masses to match the given reacting mass of the reactant

Question 15

what is atom economy?

Answer 15

a measure of the amount of reactants that end up as useful products

Question 16

% atom economy=

Answer 16

(RfM of desired product/ sum of RfM of all reactants) x100

Question 17

why is a particular reaction pathway chosen to produce a specified product, given appropriate data?

Answer 17

because the pathway chosen for a product depends on factors such as:
* percentage yield
* atom economy
* rate of reaction
* equilibrium position
* usefulness of by-products

Question 18

how to calculate the concentration of a solution when it reacts completely with another solution of a known concentration

Answer 18

1. work out how many moles of the known substance you have using no of moles= concentration x volume
2. use the reaction equation to work out how many moles of unknown substance you must have had (using ratio of moles) (e.g.if you had two moles of known and one mole of unknown in the equation and in step 1 you work out the no of moles for the known you have is 0.0025, you divide it by 2 to get the amount of moles of unknown- 0.00125
3. work out concentration of unknown using number of moles/ volume

Question 19

how is concentration of a solution in mol/dm3 related to the mass of solute and volume of solution

Answer 19

concentration can be increased by dissolving more solute in a given volume of solution - this increases the mass of the solute

Question 20

what is the volume of one mole of any gas at room temperature?

Answer 20

24 cubic decimetres (dm3)

Question 21

how to calculate the volume of a gas at room temperature and pressure from its mass and RfM

Answer 21

1. calculate moles of substance with known mass using mass/ RfM
2. using balanced equation ratio, find moles of unknown
3. calculate volume of unknown using amount in mol x molar volume (24dm3)