Amount of substance

Question 1

What is Relative Atomic Mass? (Ar)

Answer 1

⇒ (Average/mean mass of one atom of the element) / (1/12th mass of an atom of 12C)

Question 2

What is the relative molecular mass? (Mr)

Answer 2

⇒ (Average/Mean mass of one molecule) / (1/12th mass of 12C)

Question 3

How do you find the number of atoms in a substance?

Answer 3

⇒ number of atoms = moles x 6.022x10^23

Question 4

How do you find the number of molecules in a substance?

Answer 4

⇒ number of molecules = moles x 6.022x10^23

Question 5

How would you work out the mass of an isotope/element, given the Avogadro’s constant? (1)

Answer 5

⇒ MASS = Mass No1 / Avogadro’s constant
e.g: ⁷⁹Br⁺
Mass = 79/6.022 x 10²³ = 1.31 x 10⁻²² grams
Kg = 1.31 x 10⁻²²/1000 = 1.31 x 10⁻²⁵ kg

Question 6

What is the formula for concentration?

Answer 6

⇒ Concentration(mol dm^-3) = mol/volume(dm^3)

Question 7

What is the formula for concentration in terms of mass?

Answer 7

Concentration(g/dm^3)=mass(g)/volume(dm^3)

Question 8

1dm^3= xcm^3

Answer 8

⇒ 1000

Question 9

How many grams are there in 1 tonne?

Answer 9

1 tonne = 1000000grams (1x10^6)

Question 10

How many milligrams (mg) in one 1 gram?

Answer 10

⇒ 1000mg ↔ 1gram (mg↔g= ÷1000)

Question 11

What is the ideal gas equation? state its SI units.

Answer 11

⇒ pV=nRT
→ p= pressure, Pa
→ v= volume, m^3
→ n= number of moles, mol
→ R= gas constant
→ T= temperature, kelvin

Question 12

How can you convert cm^3 to m^3?

Answer 12

/1000000 (x10^6)

Question 13

How would you convert kPa and MPa into pa?

Answer 13

⇒ kPa= x1000

⇒ MPa= x1000000 (x10^6)

Question 14

1 litre=?

Answer 14

⇒ 1000cm^3 ⇒1dm^3

Question 15

What is the equation for density?

and its units

Answer 15

Density=mass/volume

⇒ Density (g cm^-3), (Kg M^3)
⇒ Mass (g/kg)
⇒ volume (cm^3), (m^3)

Question 16

What is the general method of engineering a formula?

Answer 16

⇒ Criss-cross method.
→ take into account the charges and use the criss-cross method to make and overall charge of zero.
eg: k=+1, so4=-2
→ K₂SO₄

Question 17

Write out the formula for iron(III) hydroxide.

Answer 17

Fe(III)=Fe³
Hydroxide= OH−
⇒ Fe(OH)₃ (compounds need it own individual bracket as you are mutiplying the entire bracket)

Question 18

What are the Common Positive Ions:

Answer 18

⇒ Zn²⁺ (Zinc)
⇒ Ag⁺ (Silver)
⇒ H⁺ (Hydrogen)
⇒ NH4⁺ (Ammonia)

Question 19

What are the Common Negative Ions?

Answer 19

⇒ NO₃⁻ (Nitrate)
⇒ OH⁻ (Hydroxide)
⇒ HCO₃⁻ (Hydrogencarbonate)
⇒ CO₃²⁻ (Carbonate)
⇒ SO₄²⁻ (Sulphate)

Question 20

Define empirical formula.

Answer 20

⇒ The SIMPLEST whole number ratio of atoms of each element in a compound.

Question 21

Define molecular formula.

Answer 21

⇒ The ACTUAL number of atoms of each element in a compound.

Question 22

How would we calculate the molecular formula value, needed to give the actual number of atoms, if given the relative molecular mass of the compound?

Answer 22

Relative molecular mass (given in exam question)/Mr of empirical formula

Question 23

the relative molecular mass of a compound is 284 g mol⁻¹. Given that the compound is P₂O₅. Deduce the molecular formula.

Answer 23

Mr of P₂O₅: (31x2) + (16x5) = 142
284/142 = 2
⇒ 2(P₂O₅) = P₄O₁₀

Question 24

Give 4 reasons to why 100%yield may not be achieved? (4)

Answer 24

⇒ the reaction is reversible
⇒ there are side reactions occurring
⇒ some of the product is lost when separated or through a gas.
⇒ the reaction might be incomplete.

Question 25

What is the formula for percentage atom economy?

Answer 25

⇒ (Mr of desired product/Mr of all products) x 100

Question 26

The formula for & yield:?

Answer 26

⇒ (actual yield/theoretical yield)x100

Question 27

What can a company do to maximise profits when a reaction has a low atom economy, without changing the reaction conditions or production costs?

Answer 27

⇒ Sell the product(s) to industries for financial gain.

Question 28

Why do companies in the chemical industry try to use processes with high atom economies? (4)

Answer 28

⇒ Producing little to no waste = better for the environment.
⇒ Less natural resources are used so less energy used
⇒ Less expensive
⇒ Spend less on separating the desired products from waste products and treating waste.

Question 29

State why it’s important to fill the space below the tap in the burette before beginning an accurate titration (1)

Answer 29

⇒ make sure an incorrect volume is not recorded (1)

Question 30

State why rinsing the inside of the conical flask with distilled water during a titration can improve the accuracy of the end point (1)

Answer 30

⇒ Any reagents that are stuck on the side of the flask/left over can be added to the mixture.

Question 31

State why rinsing doesn’t change the volume of solution needed in a titration or the titre value (1)

Answer 31

⇒ Water isn’t an acid or alkali so will not change the moles of the solution/reagent.

Question 32

A student rinses a BURETTE with deionised water before filling it with sodium hydroxide solution.
State and explain the effect, if any, that this rinsing will have on the value of the titre (2)

Answer 32

⇒ Titre value would increase (1)

⇒ Because the sodium hydroxide solution would be more dilute (1)

Question 33

Suggest one reason why a 250cm^3 conical flask is preferred to a 250cm^3 beaker for a titration ? (1)

Answer 33

⇒ Reduces the chance of spills when swirling the solution.

Question 34

Suggest why repeating a titration makes the value of average titre more reliable (1)

Answer 34

⇒ Titrations can give anomalous results, these can be reduced.

Question 35

Why should small amounts of a substance not be used? (1)

Answer 35

⇒ there might be errors in weighing would be too high/percentage errors too high.

Question 36

Why would large amounts of a substance not be used? (1)

Answer 36

⇒ There would be an incomplete reaction/decomposition.

Question 37

State a suitable amount of indicator solution that should be added to the flask (1)

Answer 37

⇒ 2-6 drops/ a few drops.

Question 38

State 3 ways that could improve the overall technique when carrying out a titration (3)

Answer 38

⇒ 1. Wash the glass rod into the solution to transfer any remaining substance on it. (1)
⇒ 2. Ensure the bottom of the meniscus is on the graduation mark. (1)
⇒ 3. invert the volumetric flask 2 times. (1)

Question 39

What is the hazard and precaution of using an acid?

Answer 39

⇒ Can be corrosive and harm the skin and eyes if in contact: wear safety goggles/gloves.

Question 40

What is the hazard and precaution when using a toxic substance?

Answer 40

⇒ Toxic vapours can be inhaled; carry out experiment in a fume cupboard.

Question 41

Suggest why the student should use NaOH solution rather than water for the final rinse of the burette.

Answer 41

⇒ Water will dilute NaOH and change the concentration.

Question 42

What are concordant titres?

Answer 42

⇒ Titres that are within 0.1cm3 within each other.

Question 43

What’s the formula for percentage uncertainty?

Answer 43

% uncertainty = (uncertainty/measured value)x100

e.g: The resistance of a filament lamp is given as 5.0 ± 0.4Ω. Calculate the percentage uncertainty
→ (0.4/5.0)x100 = 8%

Question 44

Weighing container with solute:10.13g
Weighing container AFTER solute added to volumetric flask: 4.48
EACH READING from the balance has an uncertainty of ±0.005g.
What is the percentage uncertainty in the mass of the solute used? (1)

Answer 44

% uncertainty = (uncertainty/measured value)x100
⇒ (0.005/(10.13-4.48))x100 = 0.09
→ Weighing scale used twice: 0.09 x 2 =0.18% (“after”)

Question 45

What is the dilution equation?

Answer 45

M₁V₁=M₂V₂
M₁: initial molarity
V₁: Initial volume
M₂: Final molarity
V₂: Final volume

Question 46

A sample has 2.0mol dm⁻³ acid has a volume of 100cm³

What volume of water, in cm³, should be added to this acid to dilute the sample to a concentration of 1.5mol dm⁻³? (1)

Answer 46

C₁V₁=C₂V₂
→ 2 x 100 = 1.5 x V₂
V₂ = (2 x 100)/1.5 = 133.3333333333...
→ The final volume = 133.333333333333
→ Initial volume = 100
Volume added = 133.333... - 100 = 33.3333333

Question 47

A student identified the use of the burette as the largest source of uncertainty in the experiment. Suggest how the procedure could be improved to reduce the %uncertainty in using the burette. (2)

Answer 47

⇒ Use a larger mass of solid (1)

⇒ So a larger titre reading will be needed (1)

Question 48

A student is required to make 250cm³ of an aqueous solution that contains a known mass. Describe the method, including apparatus and practical details, that the student should use to prepare the solution. (6)

Answer 48

⇒ : transfers known mass of solid:
→Weigh the sample bottle containing the solid on a
balance
→Transfer to the beaker and reweigh sample bottle
→Record the difference in mass (2)
⇒: Dissolves in water
→Add distilled / deionized water to the beaker
→Stir, until all solid has dissolved (2)
⇒ : Transfer washings
→ Transfer to volumetric/graduated flask with 250cm3 mark, with washings (2)
→ Make up to 250cm3 / mark with water
Shakes/inverts/mixes