3.1.2 - Amount of substance

Question 1

Define Relative Atomic Mass

Answer 1

The average mass of one atom of an element relative to 1/12th the mass of one atom of C-12

Question 2

What is the formula for Relative Formula Mass?

Answer 2

(A1 x M1) + (A2 x M2) / TOTAL ABUNDANCE

Question 3

Ideal Gas Equation

Answer 3

PV = nRT

Question 4

Conversion from oC to Kelvin

Answer 4

oC to Kelvin –> ADD 273

Question 5

What is an empirical formula?

Answer 5

The simplest ratio of atoms of each element in a compound

Question 6

What is a mole?

Answer 6

The mole is the amount of substance in grams that has the same number of particles as there are atoms in 12 grams of carbon-12

Question 7

Describe the steps of creating a Volumetric Solution

Answer 7

Common method taken from mark schemes:
1. Weigh the sample bottle containing the solid on a (2 dp) balance.
2. Transfer solid to beaker and reweigh sample bottle.
3. Record the difference in mass.
4. Add distilled water and stir with a glass rod until all the solid has dissolved.
5. Transfer to a volumetric flask with washings.
6. Make up to the 250cm3 mark with distilled water.
7. Invert flask several time

Question 8

Describe weighing by difference

Answer 8

A method to weigh materials accurately

Mass of substance transferred = mass of weighing dish and substance - mass of dish after substance has been transferred

Question 9

Why do we use a graduated volumetric flask when making standard solution?

Answer 9

Such that the solution has a known volume

Question 10

Give two ways to reduce uncertainties in measuring mass

Answer 10

1. Using a more accurate balance or a larger mass
2. Weighing by difference to calculate the actual mass of solid transferred

Question 11

If solution A is titrated against solution B, what’s in the conical flask?

Answer 11

Solution A is in the conical flask. It’s being ‘‘titrated against’’ Solution B in the burette

Question 12

What is usually in the conical flask in titration?

Answer 12

alkali

Question 13

What does concordant mean?

Answer 13

within 0.10 cm^3 of each other

means the results are accurate and reproducible

Question 14

What is the main safety precaution in titration?

Answer 14

Wear eye protection and gloves (acids and alkalis are corrosive)

Question 15

Why do we need to make sure the jet space in the burette is filled?

Answer 15

If it isn’t filled, it will lead to errors if it fills during the titration, leading to a larger than expected titre reading.

Question 16

Why do we use a conical flask over a regular beaker?

Answer 16

It is easier to swirl the mixture without spilling hte contents

Question 17

Why do we only use a few drops of indicator?

Answer 17

If too much is added, it will affect the ttration results

Question 18

Why can distilled water be used during a titration to wash the sides of the flask?

Answer 18

Water does not affect the titration as it doesn’t change the number of moles of each reactant

Question 19

How many titrations should we carry out? (at least)

Answer 19

3

Question 20

What are the colour changes for phenolphthalein and methyl orange?

Answer 20

phenolphthalein [pink (alkaline) to colourless (acid)]

methyl orange [yellow (alkali) to red (alkali)]

Question 21

When should we use phenolphthalein over methyl orange? Vice versa?

Answer 21

Use Phenolphthalein if NaOH is used

Use Methyl Orange if HCl is used

Question 22

How do we reduce uncertainties in titration?

Answer 22

1. Replacing measuring cylinders with pipettes / burettes which have lower apparatus uncertainty
2. Increasing the volume and concentration of the substance in the conical flask to increase the titre volume

Question 23

Why is a white tile used during titrations?

Answer 23

You can more easily see the colour changes and end points

Question 24

What is the empirical formula?

Answer 24

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

Question 25

What is theoretical yield?

Answer 25

The mass of product that is calculated and should have formed in a reaction

This is calculated using masses of reactants and a balanced equation

Question 26

How do you calculate percentage yield?

Answer 26

Percentage Yield = Actual Yield / Theoretical Yield x 100

Question 27

What does the percentage yield tell you?

Answer 27

Tells you how wasteful a process is, as it determines how much of the product is lost during the process

Question 28

What is atom economy?

Answer 28

A way determining how wasteful the actual reaction is, how many atoms make products that are not useful

Atom economy is a measure of the proportion of reactant atoms that become part of the desired product, rather than by-products

Question 29

How do you calculate atom economy?

Answer 29

% atom economy = molecular mass of the desired product/ sum of molecular masses of all reactants x 100

Question 30

Why is a higher atom economy more beneficial? (4)

Answer 30

• High atom economy is desirable for industrial processes
• They are better for the environment as they produce less waste
• More efficient use of raw material means high atom economy is sustainable
• Sometimes waste is expensive to dispose of safely so less waste can be cheaper / less money has to be spent separating useful product form waste products

Question 31

How can we remove water of crystallisation in compounds?

Answer 31

by heating, they can be removed as water vapour

Question 32

Devise a method to remove the water crystals from hydrated calcium sulfate crystals (6)

Answer 32

*Weigh an empty clean dry crucible and lid .
*Add 2g of hydrated calcium sulfate to the crucible and weigh again
*Heat strongly with a Bunsen for a couple of minutes
*Allow to cool
*Weigh the crucible and contents again
*Heat crucible again and reweigh until you reach a constant mass ( do this to ensure reaction is complete).

Question 33

Why shouldn’t we use small amounts of solid in experiments?

Answer 33

Percentage uncertainties in weighing will be too high

Question 34

Accuracy Questions for Water of Crystallisation Experiment:
- Why does the crucible need to be dry?
- Why do we use a lid?
- Why do we not used large amounts of hydrated calcium sulfate?
- Why don’t we use small amounts?

Answer 34

• if the lid were wet, mass loss would be inaccurate as water would be lost when heating from the crucible
• lid prevents the loss of solid from the crucible
• large amounts may mean decomposition is likely to be incomplete
• percentage uncertainty is too high

Question 35

Draw and describe the apparatus we can use to measure the volume of a gas in an experiment

Answer 35

( google doc/print )

Question 36

What are some errors involved in using a gas syringe?

Answer 36

-gas escapes before bung inserted
-syringe sticks
-some gases like CO2 are soluble in water so true amount isn’t measured

Question 37

What equation is used when changing the conditions of a gas?

Answer 37

p1v1 / t1 = p2v2 / t2

Question 38

How can we calculate volumes of gases reacting?

Answer 38

-equal volumes of under gases under same conditions will have equal moles
-therefore, ratio of volumes = ratio of moles

Question 39

What are the steps in working out what reactant is in excess

Answer 39

Step 1 - calculate amount of each reactant in mols

Step 2- use balanced equation to work out which is in excess

Step 3 - use the limiting reactant for calculations

Question 40

How do you calculate % uncertainty?

Answer 40

(uncertainty / measurement made) x 100

Question 41

Describe how we can carry out a titration. [7]

Answer 41

-Unknown [ ] Solution into conical flask using a pipette
-Known [ ] solution in burette
-Add 2 drops of indicator
-Run known solution into flask (slowly near endpoint)
-until indicator just changes colour
-Read volume from burette
-Repeat until concordant