Atoms and reactions

Question 1

Defintion of relative molecular mass, Mr

Answer 1

This compares the mass of a moleculewith 1/12th of the mass of an atom of carbon-12. It’s calculated by adding together the relative atomic masses of the elements making up the molecule.

Question 2

Defintion of relative formula mass

Answer 2

This compares the mass of a formula unit with 1/12th of the mass of an atom of carbon-12. It is calculated by adding together the relative atomic masses of the elements making up the empirical formula.

Question 3

Definition of the mole

Answer 3

The amount of (any) substance containing as many particles as there are carbon atoms in exactly 12g of carbon-12.

Question 4

Defintion of the Avogadro’s constant

Answer 4

The number of particles in one mole of any substance. It has the value of 6.02 x 10^23 mol^-1.

Question 5

Avogadro’s equ

Answer 5

Total number of particles = total number of particles in substance x Na x n

Question 6

Definition of an empirical formula

Answer 6

The simplest whole number ratio of atoms of each element in a compound.

Question 7

Definition of molecular formula

Answer 7

The actual number of atoms of each element in a compound.

Question 8

Defintion of hydrated

Answer 8

A complex staructure which has water moelcules (water of crystallisation) incorporated.

Question 9

Definition of anhydrous

Answer 9

The compound which remains when the water of crystallisation is removed from a hydrated salt.

Question 10

Definition of water of crystallisation

Answer 10

Water molecules which make up the part of the crystal structure of a compound, that can be removed by heating.

Question 11

Method for hydrated salts

Answer 11

1. Weigh an empty crucible.
2. Add the hydrated salt into the weighted crucible.
3. Weigh the crucible and the hydrated salt.
4. Using a pipe clay triangle to support the crucible containing the hydrated salt on a tripod.
5. Heat the crucible and contents gently (to prevent spitting of solution and loss of mass = inaccurate measurement) for around one minute, then heat it strongly for a further three minutes. Repeat until continuous mass.
6. Leave the crucible to cool.
7. Weigh the crucible and anhydrous salt.

Question 12

% mass =

Answer 12

(Total mass of desired chemical / total mass of the compound) x 100

Question 13

Definition of Avogadro’s law

Answer 13

At the the same temperature and pressure, equal volumes of different gases contain the same number of gas molecules.

Question 14

Definition of molar gas volume

Answer 14

The volume per mole of gas molecules at a stated temperature and pressure (dm^3mol^-1).

Question 15

Gas equ

Answer 15

Volume in dm^3 = n x 24
Volume in cm^3 = n x 24,000

Question 16

Equ for reactions not carried out at RTP

Answer 16

Pv = nRT

Question 17

Unit changes for Pv = nRT

Answer 17

Pressure: KPa –> Pa x1000
Temperature: degree –> K +273
Volume: m^3 –> dm^3 x10^6

Question 18

Concentration =

Answer 18

moles / volume

Question 19

Definition of solution

Answer 19

This is made when a solute (solid) is completely dissolved in a solvent.

Question 20

Definition of concentration

Answer 20

The amount of solute, in mol, dissolved in 1dm^3 of solvent. The units are moldm^-3.

Question 21

Definition of standard solution

Answer 21

A solution of known concentration. They are prepared by dissolving an exact mass of the solute in a solvent and making up to an exact volume using a volumetric flask (more precise).

Question 22

Why is it important to ensure the measuring of the solute is accurate?

Answer 22

If the solvent fills up above the meniscus on the volumetric flask, the volume would increase, so the moles of the solute remian the same meaning the concentration would decrease.

Question 23

Method for making up a standard solution

Answer 23

1. Accurately measure the mass of the solid carefully using a balance to an appropriate number of decimal places.
2. Dissolve the solid carefully in about 100cm^3 of distilled (or de-ionised) water, stirring with a glass rod.
3. Transfer the solution to a clean 250cm^3 volumetric flask, using a funnel.
4. Rinse the beaker, glass rod and funnel with distilled water and include the rinsings into the volumetric flask.
5. Make up to just below the mark with distilled water.
6. Make up the solution to the mark, using a dropping pipette.
7. Insert the stopper, and invert slowly several times to mix thouroughly.

Question 24

Uncertainty equ

Answer 24

(Uncertainty / measurement) x 100

Question 25

Definition of a titration

Answer 25

A technique to accurately measure the volume of one solution that reacts exactly with another solution so that the concentration of the unknown can be found.

Question 26

Burettes have an uncertainty of…

Answer 26

+/- 0.05cm^3

Question 27

Percentage yield =

Answer 27

(actual mass / theoretical mass) x 100
OR
(actual moles / theoretical moles) x 100

Question 28

Definition of percentage yield

Answer 28

How much product you actually made compared to how much you could have made in theory.

Question 29

Atom economy =

Answer 29

(sum of Mr of desired products / sume of Mr of all products) x 100