amount of substance

Question 1

complete this diagram

Answer 1

Question 2

relative atomic mass definition

Answer 2

Average mass of 1 atom of an element compared to 1/12 the mass of one atom of ¹²C which has a mass of 12

Question 3

what is the avogradro constant?

Answer 3

6.022 x 10²³ mol^-1

the number of particles in 1 mol of a substance

e.g. Carbon has a relative mass of 12 therefore 6.022 x 10²³ carbon atoms have a mass of 12g

Question 4

explain how you would show which is the limiting reactant when 100g of TiCl₄ reacts with 80g of Na using this reaction:

TiCl₄ + 4Na → 4NaCl + Ti

Answer 4

• the limiting reactant is the one that is completely used up in the reaction
• calculate the number of moles of each reactant
  
  • moles of TiCl₄ = 100/189.9 = 0.527
  • moles of Na = 80/23 = 3.48
• use the molar ratio to see which is in excess
  
  • TiCl₄ reacts with Na in a 1:4 ratio
  • therefore 0.527 mols of TiCl₄ would require 4(0.527) = 2.108 mols of Na
  • we have 3.48 mols of Na
  • therefore Na is in excess so TiCl₄ must be the limiting reactant

Question 5

an impure sample of 1.6524g barium hydroxide reacted with 100cm³ of 0.2 mol dm^-3 HCl

when the excess acid was tritated against NaOH, 10.9cm³ of NaOH was needed

25cm³ of NaOh required 28.5cm³ of HCl in a separate titration

calculate the percentage purity of the barium hydroxide sample

Answer 5

Question 6

a student is carrying out a titration

they rinsed the burette before filling it with sodium hydroxide solution

state why they should use sodium hydroxide solution rather than water for the final rinse of the burette? [1]

Answer 6

water would dilute the NaOH and change its concentration

Question 7

how to calculate percentage uncertainty for the use of the burette in a titration

Answer 7

total apparatus uncertainty / mean titre x 100

Question 8

define relative molecular mass

Answer 8

the average mass of one molecule of an element or compound relative to 1/12^th of the mass of an atom of ¹²C

Question 9

difference between percentage yield and atom economy

Answer 9

atom economy - how efficiently atoms of reactants are converted to products. takes into account waste products and doesn’t use experimental data

percentageyield- how efficient is the method. uses experimental values and does not take into account waste products

Question 10

assumptions of ideal gas equation [6]

Answer 10

• all gas molecules in constant motion
• collisions between gas molecules and walls of container create pressure
• gas molecules are very small
• mass of molecules negligible compared to the vol they occupy
• no IMF between gas molecules
• very high E_k

Question 11

784 mg of impure citric acid C₃H₅O(COOH)₃ is dissolved in water to form 250cm³ of solution

25cm³ samples were titrated with 0.05 moldm^-3 NaOH(aq)

23.95 cm³ of NaOH was required

find the mass, in mg, of citric acid dissolved in 250cm³ of the solution

the Mr of citric acid is 192

Answer 11

1 mole citric acid reatcs with 3 moles NaOH

moles NaOH = 23.95 x 0.05 x 10^-3 = 0.001198

moles acid = 0.01198 / 3 = 0.000399

mass of acid in 25cm³ = 192 x 0.000399 = 0.07664g

mass of acid in 250cm³ = 0.7664g = 766.4mg

therefore 766mg (3s.f) of citric acid was dissolved in 250cm³ of solution

Question 12

A student recorded the temperature of aqueous ethanoic acid in a polystyrene cup for three minutes.

At the fourth minute, the student added sodium hydrogencarbonate.

The student stirred the mixture and carried on recording the temperature every minute for several minutes.

The student’s measurements are shown in Figure 2. A best-fit line showing the temperature before mixing has been drawn.

Draw an appropriate best-fit line on Figure 2 and use it to find the temperature change at the time of mixing

Answer 12

draw the line of best fit as a curve by hand (do not use a ruler) and extrapolate to the 4th minute and find the temp

temp change = 17.2 - 11.4 = 5.8º

Question 13

common ionic equation

Answer 13

when ppt forms - ion(aq) + other ion (aq) → ppt (s)

neutralisation reactions : H⁺ (aq) + OH^- (aq) → H₂O (l)

acid + carbonate : 2H⁺ + CO₃^2- → CO₂ (g) + H₂O (l)

Question 14

1) write the ionic equation of the reaction between chlorine and dilute sodium hydroxide solution
2) give the reaction between chlorine and cold water

Answer 14

1) Cl₂ + 2HO^- → OCl^- + Cl^- + H₂O
2) Cl₂ + H₂O → HClO + HCl

Question 15

write ionic equations for the following:

a) reaction of aqueous potassium carbonate with nitric acid
b) precipitation of lead(II)bromide when aqueous lead(II)nitrate is mixed with aqeous sodium bromide
c) reaction of aqueous ammonia with sulfuric acid
d) reaction of hydrochloric acid with aqueous potassium hydroxide

Answer 15

identify what is changing state or charge(species that stay the same state do not appear in ionic equations)

a) carbonates react with acids to form a salt (so K+ and NO₃^- ions stay the same) ,carbon dioxide gas and water:

CO₃^2-(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l)

b) PbBr₂ ppt forms from its aqueous ions:

Pb²⁺(aq) + 2Br^-(aq) → PbBr₂(s)

c) NH₃(aq) + H⁺(aq) → NH₄⁺(aq)
d) neutralisation reactions all have the same ionic equation :

OH^-(aq) + H⁺(aq) → H₂O(l)

Question 16

describe how to prepare a burette for use

Answer 16

• rinse burette with deionised water, ensuring water flows through jet
• discard water
• rinse burette with solution you will be filling it with, ensuring it flows through the jet
• discard solution
• fill with solution

Question 17

describe how to prepare a pipette for use in a titration

Answer 17

rinse pipette with deionised water

discard water

rinse with solution you will be filling it with

discard solution

Question 18

describe how to use a pipette to measure a given volume of a solution

Answer 18

attach pipette filler to top of pipette

place pipette in solution and apply suction to draw solution up above the line on the pipette

solution is released until the bottom of the meniscus at eye level sits on the line

Question 19

describe how to carry out a dilution

Answer 19

pipette 25cm³ of the original solution into a clean volumetric flask

add deionised water to the flask until the water is just below the line

use a small disposable pipette to add deionised water very slowly until the bottom of the meniscus at eye level is on the line

stopper the flask and invert several times to mix thoroughly

Question 20

describe how to prepare a standard solution from a mass of solid

Answer 20

weigh out an accurate mass of a solid in a clean, dry beaker

add enough deionised water to dissolve the solid, stirring with a glass rod

transfer the solution to a 250cm³ volumetric flask using a funnel

rinse the beaker and glas rod with deionised water, adding the wasings to the volumetric flask

add more deionised water until the bottom of the meniscus at eye level is on the mark

stopper the flask and invert multiple times to mix thoroughly

Question 21

what kind of cemical should be used to prepare a standard solution (a solution for which the conc is accurately known)

Answer 21

has an accurately known Mr so that the number of moles can be determined (therefore hydrated salts not used)

very pure

has a high Mr so weighing errors are minimised

does not absorb moisture from or lose moisture to environment

Question 22

describe how to carry out a titration

Answer 22

• rinse burette with solution you will be filling it with, then discard rinsings and fill burette
• rinse pipette with solution you will be filling it with, discard rinsings
• use the pipette and a pipette filler to transfer 25cm³ of this solution into a conical flask
• add 2-3 drop of indicator to the conical flask
• add the solution from the burette, constantly swirling the conical flask, until the solution in the flask just changes colour. this is the rough titration
• to reduce the effect of random error on titration results, repeat until you achieve 2-3 concordant results, making sure you add the solution from the burette dropwise near the end point
• calculate the mean titre from the concordant results (within 0.1 cm³ of each other)

Question 23

a burette has uncertainty ±0.05 cm³

in a titration the initial burette reading was 0.05cm³ and the final burette reading was 22.55cm³

what is the percentage uncertainty in the titre value?

Answer 23

titre value = 22.55 - 0.05 = 22.5

percentage uncertainty = 2 x 0.05/22.5 x 100 = 0.44%

remember to multiply by two because you are using two readings

Question 24

some of the solution from the burette may run down the inside of the conical flask

how can you ensure all the solution added from the burette has a chance to react (apart from swirling the conical flask)?

Answer 24

squirt deionised water down the side of the flask to was the solution from the burette down into the solution below

the amount of moles present in the pipetted solution will no change so this will not affect the titre achieved

Question 25

in a titration, KOH is added to a conical flask

3 drops of methyl orange indicator are added and HNO₃ is added from a burette until the indicator changes colour

which of the following would lead to the titre being larger than it should be?

A rinsing the conical flask with water before adding KOH

B rinsing the burette with water before filling it with HNO₃

C rinsing the pipette with water before filling it with KOH

D adding extra drops of indicator

Answer 25

rinsing the pipette/conical flask with water would dilute the KOH so less HNO₃ would be needed for neutralisation = smaller titre value

adding extra drops of indicator has no effect

rinsing the burette with water will dilute the HNO₃ so more would need to be added for neutralisation = greater titre value

therefore B is the correct answer

Question 26

Answer 26

Question 27

describe how you would dilute 25cm³ of oven cleaner to 250cm³ and transfer 25cm³ of the diluted solution to the flask

Answer 27

Question 28

why should you remove the funnel from the burette before a titration?

Answer 28

to ensure any drops of the solution which remain in the funnel do not interfere with titration results