C4 - Chemical Calculations

Question 1

What is the relative formula mass of a substance

Answer 1

The sum of the relative atomic masses of all the atoms shown in its formula

Question 2

Equation for the percentage mass of an element in its compound

Answer 2

percentage mass of element in compound = (Ar x number of atoms of element) / Mr of the compound X 100

Question 3

What is the avogadro number of particles

Answer 3

6.02 x 10^23

Question 4

Equation linking mass Mr and moles

Answer 4

Mass (g) = Mr x Number of moles

Question 5

Reasons for mass changing in a reaction

Answer 5

1. Mass increases if one of the reactants is a gas and all products are solids, liquids or aqueous
2. Mass decreases if one of products is gas and all reactants are solids, liquids or aqueous

Question 6

What is the limiting reactant

Answer 6

One of the reactions which gets used up and causes a reaction to stop

Question 7

What is an excess reactant

Answer 7

A reactant added in excess to make sure the other (limiting) reactant is used up

Question 8

How to calculate mass of product formed by using the limiting reactant and balanced reaction equation

Answer 8

1. Work out the Mr of reactant and product you want
2. Find out how many moles there are of substance you know the mass of (Mol = mass/mr).
3. Use balanced equation to work out how many moles there will be of the other substance
4. Use number of moles to calculate the mass

Question 9

Rules for volume of gas

Answer 9

At the same temperature and pressure, equal numbers of moles of any gas will occupy the same volume. One mole of any gas occupies 24dm^3 or 24000cm^3

Question 10

Equation linking volume of gas to mass and mr

Answer 10

Volume of gas (dm^3) = mass of gas / mr of gas X 24

Question 11

Equation linking moles of gas to volume

Answer 11

Moles = volume (dm3) / 24 dm3

Moles = volume (cm3) / 24000cm3

Question 12

what is the concentration of a solution

Answer 12

The amount of a substance in a certain volume of solution. The more solute, the more concentrated the solution

Question 13

Equation for concentration

(g/dm3)

Answer 13

Concentration (g/dm) = amount of solute (g) / volume of solvent (dm3)

Question 14

Equation for concentration (mol/dm3)

Answer 14

concentration = amount of solute (mol) / volume of solvent (dm3)

Question 15

How to get to from dm3 to cm3

Answer 15

x 1000

Question 16

How to get from cm3 to dm3

Answer 16

divide by 1000

Question 17

How to get from (mol/dm3) to (g/dm3)

Answer 17

times by the Mr

Question 18

How to increase the concentration for an aqueous solution

Answer 18

1. Add more solute and dissolving it in the same volume of solvent
2. Evaporate off some of solvent so you have the same mass of solute in a smaller volume of solvent

Question 19

What are titrations used for

Answer 19

to determine the volume of acid and alkali that react together completely to form a neutral salt solution.
OR

to determine the concentration of an acid or alkali of the concentration of the reactant is known

Question 20

Method for titrations

Answer 20

1. unknown concentration (usually alkali) is added to a conical flask. usually 25cm3 is added using a pipette to measure
2. Add a few drops of phenolpthalein indicator to the solution
3. Use a funnel to fill a burette with (acid) of known concentration. Do below eye level and wear safety goggles. Record the initial volume in burette
4. Use burette to add the known (acid) solution to the unknown, slowly, giving the conical flask a regular swirl
5. When indicator changes colour (from pink -alkali- to colourless -acidic-) the (alkali) has been neutralised
6. Record the final volume of the burette and use it, along with initial reading, to calculate the volume of acid needed to neutralise the alkali. Use solution calculation

Question 21

Considerations for titration

Answer 21

• A white tile should be used to observe colour change
• A test run should be performed first to give a rough idea of how much acid or alkali should be added. After this, a more accurate titration should be done
• Repeat until you get consistent results (within 0.1cm3 of each other)

Question 22

What does the atom economy of a reaction tell you

Answer 22

How much of the mass of reactants is wasted when manufacturing a chemical and how much ends up as the desired product.
100% atom economy means all atoms in reactants have turned into the desired product.
The amount of reactants that end up as useful products is called the Atom Economy

Question 23

Disadvantages of reactions with a low atom economy

Answer 23

They are unsustainable and make lots of waste products which can also be expensive to remove and dispose of responsibly. Pollution can also be caused by waste products.

Question 24

Equation for atom economy

Answer 24

atom economy (%) = Mr of desired product / sum of Mr of all reactants X 100

Question 25

What does the percentage yield of a chemical reaction tell you

Answer 25

How much product the reaction actually produces compared with the maximum amount that could possibly be produced (100%)

Question 26

Equation for percentage yield

Answer 26

Percentage yield = mass of product produced / maximum theoretical mass of product X 100

Question 27

Reasons for NOT getting a 100% yield

Answer 27

• Reaction is reversible
• Some reactants may react to give unexpected or unwanted products in alternative reactions
• some of product may be lost in handling or left on the apparatus
• Reactants may not be pure
• Some of desired product may be lost during the separation from its reaction mixture