Amount of Substance

Question 1

Define Relative Atomic Mass

Answer 1

The average mass of an atom of an element (taking into account its naturally occurring isotopes) relative to 1/12th of the relative atomic mass of a carbon-12 atom.

Question 2

What is the Avogadro Constant?

Answer 2

6.022 × 10^23

Question 3

Define Relative Molecular/Formula Mass

Answer 3

The average mass of a molecule/compound (taking into account its naturally occurring isotopes) relative to 1/12th of the relative atomic mass of a carbon-12 atom.

Question 4

What is the equation involving moles, mass and Mr?

Answer 4

Moles = Mass/Mr

Question 5

What is the equation involving moles, concentration and volume?

Answer 5

Concentration = Moles/Volume

Question 6

What is the Ideal Gas Equation?

Answer 6

PV = nRT
P - Pressure (Pascals)
V - Volume (m3)
n - Moles
R - Gas Constant (8.31)
T - Temperature (Kelvin)

Question 7

What is the empirical formula?

Answer 7

The whole number ratio of elements in a compound, may not be the actual amount of elements in the compound.

Question 8

What is the molecular formula?

Answer 8

The true number of each element in the compound.

Question 9

How do you calculate the molecular formula?

Answer 9

Divide the ACTUAL Mr of the compound by the Mr of the EMPIRICAL FORMULA. Use this multiplier to scale up the empirical formula.

Question 10

How do you calculate empirical formula?

Answer 10

1. Find the mass or % of each element in the compound.
2. Divide each value by the relative atomic mass of its element.
3. Put them into a ratio and divide all values by the smallest number (in order to have a 1).
4. IF RATIO, close to 0.5, multiply by 2, close to 0.34/0.33, multiply by 3.

Question 11

What is the equation for Percentage Yield?

Answer 11

100(Actual Mass Made/Theoretical Mass)

Question 12

What is the equation for Atom Economy?

Answer 12

100(Mr of desired product/Mr of reactants)

Question 13

Give an economic, environmental and ethical advantages for having a high atom economy.

Answer 13

Little to no waste product, means less money wasted and less natural resources used up (often less energy)

Question 14

REQUIRED PRACTICAL 1

Answer 14

Make up a volumetric solution and carry out a simple acid–base titration.

Question 15

What is the method for making a standard solution?

Answer 15

1. Measure mass of dry weighing boat on weigh scale. Use a spatula to add the known mass of the soil. Measure mass of weighing boat with solid. To 2 d.p.
2. Put solid from weighing boat into beaker and reweigh empty boat again (weight of weighing boat with solid - weight of weighing boat alone = mass of solid added to beaker).
3. Add a little bit of deionised water to dissolve solid and stir with stirring rod.
4. Transfer solution to a volumetric flask with the funnel.
5. Add deionised water with pipette (dropwise at end) until the meniscus reaches the line.
6. Put stopper on volumetric flask and invert a few times to ensure proper mixing.

Question 16

How do you prepare the burette and pipette?

Answer 16

1. Rinse with deionised water, going down all sides.
2. Rinse with respective solutions (either standard solution in pipette or acid/base going into the burette), going down all sides.
3. Fill burette and pipette with respective solutions - avoid air bubbles in tip of apparatus!

Question 17

What is the method for the titration?

Answer 17

1. With a pipette, add a given volume of the standard solution into the conical flask.
2. Add the base/acid into the burette with a funnel until the meniscus reaches 0 (or another value).
3. Add 2-3 drops of the indicator and place on a conical flask under burette on a white tile.
4. Turn the tap and allow acid/base to pour into conical flask, consistently swirling.
5. Turn off tap when colour change occurs, record the volume of acid/base used reading meniscus at eye level, this is your rough titration (2 d.p.).
6. Repeat above steps but go dropwise near the rough value to observe the colour change at the exact drop.
7. Repeat this until you get concordant results, within 0.1cm^3.

Question 18

How are conical flasks prepared?

Answer 18

Rinse in deionised water, do not need to dry between uses. Deionised water has no effect on exact moles of solute or volume of solution added so DO NOT RINSE WITH STANDARD SOLUTION.

Question 19

How would an air bubble in the burette or pipette affect the results?

Answer 19

This would give a slightly higher titre value because volume in apparatus would be less than estimated.

Question 20

Why do you remove the funnel in the burette during titrations?

Answer 20

A drop of acid/base may go into the burette, increasing the volume than previously estimated, decreasing the titre value.

Question 21

Why are conical flasks used?

Answer 21

Easy to swirl since sides prevent sloution from spilling out.

Question 22

What is an uncertainty?

Answer 22

Estimate of the measurement or reading and the range of values within which the true value must be.

Question 23

How do you know when to use value uncertainties or percentage uncertainties?

Answer 23

Adding or subtracting measurements or values means their uncertainties can be added.
Dividng or multiplying measurements or values means their percentage uncertainties must be calculated, then added.

Question 24

How do you calculate uncertainty? How do you then calculate percentage uncertainty?

Answer 24

Uncertainty is usually written on apparatus used.
Percentage uncertainty = 100(Uncertainty/Measured Value)

Question 25

How do you decrease percentage uncertainty?

Answer 25

• Increase the size of the measurement made (more d.p. or decrease conc. of acid/base in burette or increase vol. and conc. of standard solution).
• Use apparatus with a greater resolution (more scale divisions) or smaller uncertainty.
• Use pipette, not measuring cylinder.
• Use burette.
• Weigh solid sample before and after addition to burette to calculate exact mass added.