Analysing data from pH measurements

Question 1

the relative strength of different acids can be determined by

Answer 1

measuring the pH of equimolar aqueous solutions of the acids, at the same temperature

Question 2

the higher the value of the pH, the ……………the acid

Answer 2

weaker

Question 3

the relative strength of different bases can be determined by

Answer 3

measuring the pH of equimolar aqueous solutions of the bases, at the same temperature

Question 4

the higher the value of the pH, the………………the base

Answer 4

stronger

Question 5

the pH of NaCl is…..because

Answer 5

neutral (7.00) because the salt is made from a strong acid (HCl) and a strong base (NaOH)

Question 6

the pH of KNO3 is…..because

Answer 6

neutral because it is a product of the strong acid HNO3 and the strong base KOH

Question 7

an aqueous solution of CH3COONa is……………..because

Answer 7

alkaline because it is a product of the weak acid CH3COOH and the strong base NaOH

Question 8

an aqueous solution of NH4Cl is……………..because

Answer 8

acidic because it is a product of a strong acid (HCL) and a weak base NH3

Question 9

for strong acids, the pH increases by a factor of…………unit(s) for each 10-fold decrease in concentration

Answer 9

1 unit

Question 10

for weak acids, the pH increases by a factor of about ………….unit(s) for each 10-fold decrease in concentration

Answer 10

0.5 units

Question 11

the steps for the experiment whereby the Ka of benzoic acid (C6H5COOH) is found is:

Answer 11

• weigh 0.4-0.5g of benzoic acid and then dissolve it in a small volume (50cm3) of deionised water in a beaker. you may need to warm the water as benzoic acid is not very soluble, if so allow to cool before performing next step
• transfer solution to 250 cm3 volumetric flask. add several wask=hings from beaker then make up to mark using deionised water
• mix the solution by inverting
• withdraw a sample of the solution and place it in a small beaker
• using a calibrated pH meter, measure the pH of the solution

Question 12

0.49g of benzoic acid was used and the pH found was 3.00. was is the Ka for benzoic acid at 298K?

Answer 12

-C6H5COOH ⇌ C6H5COO- + H+
- molar mass of benzoic acid is 122 g mol-1
- 0.49g / 122 = n of C6H5COOH in 250 cm3 of solution
-so, [C6H5COOH(aq)] = (0.49/122) x 4 mol dm-3
-pH= 3.00
-so, [H+(aq)] = 10(-3.00) = [C6H5COOH]
Ka = (10(-3.00))2 / ((0.49/122)x4)
=6.22 x 10(-5) mol dm-3