Quantitative analysis

Question 1

How do you calculate the concentration of a solution

in mol dm^-3?

Answer 1

Concentration (mol dm^-3) = moles ÷ volume (dm^3)

Question 2

How can you convert concentration in g dm^-3 to mol dm^-3?

Answer 2

Concentration(mol dm^-3) = concentration(g dm^-3) ÷ Mr

Mr - molecular mass

Question 3

Describe how to carry out an acid-alkali titration to

find the concentration of alkali

Answer 3

1. Rinse the pipette with the solution of unknown concentration (alkali). Use
   this pipette to add a known volume of alkali to a conical flask.
2. Add a few drops of indicator.
3. Rinse and fill the burette with acid.
4. Gradually add the acid to the conical flask.
5. Record the volume in the burette when the indicator changes colour.
6. Repeat the experiment until you have concordant results.
7. Use results to calculate a mean titre and the concentration of alkali.

Question 4

If you know the volume of acid required to neutralise an
alkali, how could you calculate the concentration of the acid,
given the alkali concentration and volume?

Answer 4

• Calculate the number of moles of the alkali using the known volume
  and concentration (moles = concentration x volume).
• Use the chemical equation to work out the ratio of acid and alkali
  that react and hence work out how many mole of acid have reacted.
• Divide the moles of acid by the volume used in neutralisation to find
  concentration.

Question 5

What is meant by the term theoretical yield?

Answer 5

The amount of product that would be
collected under perfect reaction
conditions.

Question 6

How do you calculate percentage yield?

Answer 6

Percentage yield =

(Actual yield ÷ Theoretical yield) x 100

Question 7

What is the percentage yield of NH3

if 40.5 g of NH3 is produced from 20.0 mol H2 and excess N2?

Answer 7

1. Equation: N2
   + 3H2 → 2NH3
2. Moles of ammonia = 20/1.5 = 13.3 moles
3. Mass of ammonia = 13.3 x (14+1+1+1) = 227 g
4. Percentage yield = (40.5/227) x 100 = 17.8%

Question 8

Why could the actual yield of product be less than

expected?

Answer 8

• Incomplete reaction.
• Competing, unwanted side reactions.
• Practical losses, for example some solid
  may get lost when being transferred
  between beakers.

Question 9

What is the atom economy of a reaction?

Answer 9

Atom economy is a measure of the efficiency of the reaction. It looks at the amount of reactants that get turned into useful products.

Question 10

How can atom economy be calculated?

Answer 10

Atom economy =
(Mr of desired product ÷ Mr of reactants) x 100

Mr - molecular mass

Question 11

Fill in the gap: ‘The _____ the atom
economy, the more sustainable and
efficient the process’

Answer 11

Higher

Question 12

Why might one reaction pathway be chosen over

another?

Answer 12

• Higher atom economy.
• Higher yield.
• Faster rate.
• Equilibrium position favours products more.
• By-products are more useful / less harmful.

Question 13

What does Avogadro’s Law state about gas

volumes?

Answer 13

At the same temperature and pressure,
equal amounts of gas will occupy the
same volume

Question 14

What is the molar volume of a gas?

Answer 14

The volume occupied by one mole of
molecules of any gas at room
temperature and pressure.

Question 15

What is RTP?

Answer 15

Room temperature and pressure:

• 20°C
• 1 atmosphere

Question 16

How can the molar volume of a gas be calculated?

What are the units?

Answer 16

Molar volume = volume (dm^3) ÷ moles of gas

Units: dm^3 mol^-1

Question 17

What is the unit of concentration?

Answer 17

mol dm^-3