Moles 2 - 4th and 5th

Question 1

what do we usually measure volumes of gas in?

Answer 1

• cubic centimetres (cm^3)

Question 2

what unit is commonly used in mole calculations?

Answer 2

• cubic decimetres (dm^3)

Question 3

how do you convert between centimetres and decimetres?

Answer 3

• cm^3 -> dm^3 = /1000
• dm^3 -> cm^3 = x1000

Question 4

how can the moles of a gas be calculated?

Answer 4

• from its volume alone
• moles = volume(dm^3) / 24dm^3
• the denominator is 24dm^3 for EVERY gas
  OR
• moles = volume(cm^3) / 24000(cm^3)
• the denominator is 24000cm^3 for EVERY gas

Question 5

calculate the amount in moles in 1500cm^3 of H2.

Answer 5

• moles = volume(cm^3) / 24000(cm^3)
• 1500/24000
• 0.0625 mol

Question 6

calculate the amount in moles in 6dm^3 of CO2.

Answer 6

• moles = volume(dm^3) / 24dm^3
• 6/24
• 0.25mol

Question 7

calculate the volume of 0.8 moles of CH4.

Answer 7

• volume (dm^3) = moles x 24dm^3
• 0.8 x 24
• 19.2 dm^3

Question 8

what steps do you need to do in order to calculate the volume of O2 needed to react with 10g of N2?

Answer 8

• N2 + 2O2 -> 2NO2
  1. convert the mass of N2 into moles of N2
  2. use the mole ratio from the balancing numbers in the equation.
  3. convert moles of O2 into the volume of O2

Question 9

what steps do you need to do in order to calculate the volume of NO2 produced from 5dm^3 of N2?

Answer 9

• N2 + 2O2 -> 2NO2
  1. use the mole ratio from the balancing numbers in the equation

Question 10

what are the 3 different formulae for moles and in which circumstances do you use each one?

Answer 10

• mass(g) / RFM - USE THIS FOR ANYTHING
• volume (dm^3) / 24dm^3 - FOR GAS VOLUMES
• moles of solute/volume (dm^3) of solvent - USE THIS TO FIND OUT THE CONCENTRATION OF A SOLUTION IN MOL/DM^3

Question 11

what is the concentration of a solution a measure of?

Answer 11

• how much solute is dissolved per unit volume

Question 12

what is the concentration of a solution measured in and what is the range for it?

Answer 12

• measured in mol/dm^3
• usually varies between 0.1 and 2.0

Question 13

calculate the concentration when 0.028 moles dissolves into 50cm^3.

Answer 13

• convert cm^3 to dm^3, rearrange to find moles and then stick the numbers in
• conc (mol/dm^3) = moles / volume (dm^3)
• = 0.028/ (50/1000)
• = 0.56 mol/dm^3

Question 14

calculate the moles contained in 230cm^3 of a 0.15 mol/dm^3 solution.

Answer 14

• rearrange to find volume, then stick the numbers in
• vols (dm^3) = moles / conc (mol/dm^3)
• = 0.2 / 1.5
• = 0.133 dm^3

Question 15

what are the two special pieces of equipment used in titrations?

Answer 15

• volumetric pipette - measures out 25cm^3 very accurately. hopeless at measuring out any other volume
• burette - measures any volume to the nearest 0.05 cm^3

Question 16

what is titration used for?

Answer 16

• a practical method used to measure the volume (or ‘“titre”) of acid needed to neutralise a particular volume of alkali, or vice-versa

Question 17

what does the procedure of titration assume?

Answer 17

• the acid is known, so it goes in the burette
• the alkali is unknown, so it goes into the conical flask

Question 18

describe the setup of the titration procedure.

Answer 18

setup:
1. use a volumetric pipette to place 25cm^3 of the unknown alkali into a conical flask
2. add a few drops of either methyl orange or phenolphthalein indicator to the flask. universal indicator doesn’t work because it changes colour too gradually
3. fill a burette with known acid and clamp it above the conical flask

Question 19

describe the usage of the titration procedure?

Answer 19

4. measure the initial volume of known acid on the burette, to the nearest 0.05cm^3
5. use the burette to add acid into the conical flask drop-by-drop and swirling the flask
6. close the burette tap as soon as the colour in the conical flask changes. the start colour will be the alkaline colour, the end colour will be the acidic colour (because the last drop of acid always has a tiny amount more than is needed to neutralise).
7. measure the final volume of known acid on the burette, to the nearest 0.05cm^3
8. calculate titre (final volume- initial volume)
9. repeat to get concordant (within 0.2cm^3) titres

Question 20

what are the results of the titration procedure?

Answer 20

• you record your results in a table , with every volume to the nearest 0.05 cm^3
• with initial vol. , final vol. and titre on the left and expt 1, 2 and 3

Question 21

how do you calculate the mean titre?

Answer 21

• only calculated based on the concordant tires (within 0.2cm^3 of each other
• add them up and divide by how many there are

Question 22

how can you use the titration results in mole calculations?

Answer 22

• use the mole ratio in the balancing numbers from the reaction equation to convert from moles of a substance you know lots about (e.g. the known acid) to moles of a substance you don’t know much about (the unknown alkali)

Question 23

calculate the concentration of NaOH:
- in a titration, jenny measured that 20.675cm^3 of 0.1 mol/dm^3 H2SO4 was required to neutralise 25cm^3 of NaOH solution.

Answer 23

STEP ONE : grid out the numbers in the question
NaOH H2SO4
conc (mol/dm^3) ????? 0.1
vol (cm^3) 25 20.675

STEP TWO: use your grid to help you do a normal reacting moles calculation

1. calculate the moles of H2SO4
2. Use the mole ratio from the equation to get the moles of NaOH
3. calculate the concentration of the NaOH