Titrations

Question 1

Antacid equation

Answer 1

Mg(OH)₂ + 2HCl ➞ MgCl₂ + 2H₂O

Question 2

Titration equation

Answer 2

NaOH + HCl ➞ NaCl + H₂O

Question 3

Group 2 trend in atomic radius

Answer 3

Increases down group - extra electrons shells down group

Question 4

Group 2 trend in ionisation energy

Answer 4

Decreases down group - more shielding, distance from outer shell to nucleus

Question 5

Reactivity of Group 2

Answer 5

Increases down group - easier it is to lose electrons, more reactive the element

Question 6

Trend in melting point in Group 2

Answer 6

• **Decreases **except for Mg
• Metallic ions get bigger
• # of delocalised electrons same but further away from nucleus
• Reduced attraction betwen nucleus + sea of delocalised electrons

Question 7

Trend in solubility down Group 2

Answer 7

• OH^- hydroxide ions increase (single - charge)
• SO₄^2- sulfate ions decrease (double - charge)

Question 8

% atom economy

Answer 8

( mass of useful products / mass of all reactants ) x 100

Question 9

Apparatus error

Answer 9

( 100 x margin of error ) / quantity measured

Question 10

% yield

Answer 10

( actual yield / theoretical yield ) x 100

Question 11

Why use a conical flask rather than a beaker?

Answer 11

Easier to swirl without spilling its contents

Question 12

Why doesn’t the Mr need to be exact to get the molecular formula from the empirical?

Answer 12

Because the molecular formula will be a whole number multiple of the empirical fomula

Question 13

Why do you need to note down temp. and pressure?

Answer 13

Because the volume of a gas depends on pressure and temperature

Question 14

Potential errors in using a gas syringe?

Answer 14

• Gas escapes before bung is inserted
• Syringe sticks
• Some gases are soluble in water so true amount not measured (eg SO₂ or CO₂)

Question 15

Experimental error %

Answer 15

100 x (real answer - experimental answer) / real answer

Question 16

Experimental error > Apparatus error

Answer 16

Results deemed to be inaccurate

• Discrepancy
• Result due to other errors

Question 17

Why do we need concordant results?

Answer 17

So we can say results are accurate and reproducaible and titration technique is good/consistent

Question 18

Why is distilled water used to wash the sides of the flask?

Answer 18

• To ensure all acid on the side is washed into reaction mixture
• Water doesn’t react with reagents of change number of moles

Question 19

Why is Mg(OH)₂ suitable for dealing with excess stomach acid?

Answer 19

• Low solubility in water
• Not corrosive as it is a weak alkaline
• Doesn’t produce CO₂ gas (which can cause wind)

Question 20

How can you decrease sensitivity error?

Answer 20

• Increase size of measuremenet made
• Use apparatus with a greater resolution

Question 21

How can you reduce error in a burette reading?

Answer 21

Increase titre volume by:

• increasing vol. and conc. of substance in flask
• decreasing conc. of substance in burette

Question 22

How can you reduce error in measuring mass?

Answer 22

Weighing sample before and after addition, then calculating the difference

Question 23

Why should we fill the jet space in the burette?

Answer 23

May lead to a larger than expected tire reading

Question 24

Experimental error (% diff. between actual and calculated value) < Apparatus error

Answer 24

• All errors can be explained by sensitivity of equipment
• No discrepancy

Question 25

Phenolphthalein

Answer 25

Acid - colourless

Bases - pink

Question 26

Reaction equation

Answer 26

HCl (excess - 250cm³) = HCl (neut. by tablet) + HCl (neut. by NaOH)