Chapter 4 - Chemical Calculations

Question 1

What is the Relative Atomic Mass and the Relative Formula Mass?

Answer 1

The Relative Atomic Mass is the relative mass of atoms of different elements. The Relative Formula Mass is a mass calculated using the Relative Atomic Mass of different elements.

Question 2

How do you calculate the number of moles in a substance?

State the Avogadro’s Constant.

Answer 2

mass (g)
———-
Ar / Mr

Avogardo’s Constant = 6.02 x 10 (23) per mole

Question 3

What is the limiting reactant in a reaction?

Answer 3

The limiting reactant in a reaction is the reactant that gets used up first.

Question 4

How do you calculate the percentage yield of a chemical reaction?

Answer 4

Actual Yield Produced
———————————– x 100%
Maximum yield possible

Question 5

What are the factors affecting percentage yield?

Answer 5

• Product being left behind in the apparatus
• Reversible reactions not being completed
• Reactants may produce unexpected reactions
• Loss from separating products from the reaction mixture
• Reactant may not be pure

Question 6

How do you calculate the atom economy of a substance?

Answer 6

Sum of reactants (from equation)

Question 7

How do you calculate the concentration of a substance?

Answer 7

amount of solute (g)
—————————— ( x1000 to converse dm3 to cm3)
Volume of Solution (dm3)

Concentration in mol/dm3

Question 8

RP - Describe how to carry out a Titration between an acid and an alkali substance (6)

Answer 8

1. Measure a known volume of about 25cm3 of alkali or acid into a conical flask using a pipette filler.
2. Add a few drops of acid indicator to the solution.
3. Fill a burette with your chosen acid or alkali of about 50 cm3.
4. Record the reading on the burette, then open the tap slowly (dropwise) to release the acid. Swirl the flask as the acid falls into the conical flask.
5. Repeat step 4 until the alkali changes colour. Immediately close the tap and record the reading on the burette.
6. Repeat these steps at least 3 times then calculate a mean to give an accurate result.

Question 9

How do you calculate the moles of gases?

What is the volume of 1 mole of any gas at room temperature? (dm3)

Answer 9

Volume of Gas (dm3)
________________

         24dm3

• The volume of 1 mole of any gas at room temperature is 24 dm3.

Question 10

How do you calculate the concentration of a substance with titrations?

Answer 10

Number of Moles
____________

Volume of Solution (dm3)

• Balance any equations if necessary, as this will tell us the ratio needed if there is one present.
• Use the number of moles, usually given in the question.
• Use given results from a table or from other sources of information as the volume. Divide by 1000 to give the volume in dm3.

Question 11

How do you calculate the concentration for unknowns?

Answer 11

1. Calculate the number of moles in 25cm3 of acid of (e.g 0.4M NaOH). Convert cm3 to dm3 by dividing by 1000. Use the titration concentration equation and rearrange it.
2. If required, balance the equation for the reaction.
3. Identify the number of acid moles needed to neutralise the alkali moles (e.g a 1:1 ratio)
4. Use the number of moles of acid and the mean volume used (calculated from a results table) to calculate the concentration of the acid. Use the titration concentration equation.

Question 12

A student performs a titration between sulfuric acid (H2SO4) and 25cm3 of 0.5M Sodium Hydroxide (NaOH)

Calculate the concentration of the sulfuric acid used to neutralise the sodium hydroxide

Use the formula:

 Number of moles
--------------------------                  = Concentration (mol/dm3) Vol of solution (dm3)

Answer 12

1. Moles of NaOH:

25/1000 = 0.025
mol = 0.5 x 0.025
mol = 0.0125

Balanced equation:

H2SO4 + 2NaOH = 2NaSO4 + 2H2O
1:2 ratio formed
0.0125 NaOH mol = 0.00625 H2SO4 mol

Concentration of acid:

26.6
——– = 0.0266
1000

Concentration = 0.00625 / 0.0266
Concentration = 0.23M