1.12 Acids & bases

Question 1

Define weak acid
ii) define a strong acid

Answer 1

A proton donor that
i) partially dissociates
ii) fully dissociates

Question 2

Calculate the pH for a solution of HCl with a concentration of 0.064. Give your answer to 2 decimal places.

Answer 2

pH = -log{H^+}
= 1.19

Question 3

Calculate the pH for a solution of NaOH with a concentration of 0.157. Give your answer to 2 decimal places.
Kw at 25 oC = 1 x 10-14 mol2 dm-6

Answer 3

Since NaOH is a strong base, it dissociates completely in water:
NaOH → Na^+ + OH^−
This means the concentration of hydroxide ions, is equal to the concentration of NaOH, which is 0.157 mol/dm³
calculate pOH=−log(0.157) = 0.803
pH+pOH=14(at25°C)
14-0.803 = 13.20

Question 4

What is a Lowry Bronsted base

Answer 4

proton acceptor

Question 5

Calculate the pH for a solution of Ba(OH)2 with a concentration of 0.21. Give your answer to 2 decimal places.
Kw at 25 oC = 1 x 10-14 mol2 dm-6
b) show method using Kw

Answer 5

Ba(OH) 2 →Ba ^2+ +2OH ^−
concentration of OH^- = 0.21 x 2 = 0.42
pOH = -log(0.42) = 0.376
pH = 14 - 0.376 = 13.62
b) Kw = {OH}^- {h}^+
sub values in, use - log to get answer of 13.62

Question 6

What type of acids are CH3COOH & HCOOH

Answer 6

Weak acids

Question 7

A beaker contains 100.0 cm–3 of 0.0125 mol dm−3 nitric acid.
Calculate the pH of the solution formed after 50.0 cm3 of 0.0108 mol dm−3 aqueous sodium hydroxide are added to the beaker.
Give your answer to 2 decimal places.

Answer 7

HNO 3(aq) + NaOH(aq)→NaNO3(aq)+H2O(l)
moles of HN03 = 0.1 x 0.0125 = 0.00125 mol
moles of NaOH = 0.0108 x 0.05 = 0.00054 mol
- NaOH is limiting reagent as moles of HNO3 left = 0.00125 - 0.00054 = 0.00071 mol
total volume = 0.1 + 0.05 = 0.15 dm^3
concentration = 0.00071 / 0.15 = 0.00473
pH = -log (0.00473) = 2.326 = 2.33

Question 8

Define pH

Answer 8

pH = -log10{H^+}

Question 9

Chemical equation for nitric acid + potassium oxide

Answer 9

2HNO3 (aq) + K2O (s) -> 2KNO3 (aq) + H2O (l)

Question 10

Chemical equation for phosphoric acid + sodium hydroxide

Answer 10

H3PO4 (aq) + 3NaOH (s) -> Na3PO4 (aq) + 3H2O (l)

Question 11

Suggest why the pH probe is washed with distilled water between each of the calibration measurements.

Answer 11

Different solutions must not contaminate each other /
to wash off any substance that could affect the reading

Question 12

The calibrated pH meter is used to monitor the pH during a titration of hydrochloric acid with sodium hydroxide.
Explain why the volume of sodium hydroxide solution added between each pH measurement is smaller as the end point of the titration is approached.

Answer 12

To avoid missing the end point

Question 13

state why a certain indicator will be suitable for a certain titration

Answer 13

all have a colour change within the vertical part of the titration curve