Pre-Laboratory Assignment 1. Briefly explain a) why you must use caution when yo

Question

Pre-Laboratory Assignment 1. Briefly explain a) why you must use caution when you work with HCl solutions. b) how you are going to process for disposal the HC,H02 solutions you have discarded. 2. Briefly distinguish between the following terms, as they pertain to this experiment. (a) strong aclds and strong bases (b) strong acids and weak aclds 3. Calculate (a) the pH of a solution in which [H,0]-1.5x10-M (b) the pH of a solution in which (OH1-3.1 x 10-M (c) [H O*] when pH-8.42 (d) [HO] when pOH-5.58

Explanation / Answer

1. a. Concentrated HCl solution that we work with in laboratories is a highly corrosive acid and can cause severe burns to the skin. It has a strong, pungent odour which is harmful to breathe. On coming in contact with metals it releases hydrogen gas which is highly flammable and might lead to explosions and hence needs to be handled with care.

b. It may be poured down the sink. Then wash the area with water. CH3COOH i.e acetic acid is a weak acid and is not very corrosive and can be easily rinsed off with water.

2. a. Strong acids are those which completely dissociate when they are dissolved in water to release H+ ions in water.

Eg. HCl is a strong acid since in water HCl dissociates completely into H+ and Cl-.

Strong bases are those which completely dissociate when they are dissolved in water to release OH- ions in water.

Eg. NaOH is a strong base since in water NaOH dissociates completely into OH- and Na+.

b. Strong acids are those which completely dissociate when they are dissolved in water to release H+ ions in water. Hence a strong acid solutions consists of only ions.

Eg. HCl is a strong acid since in water HCl dissociates completely into H+ and Cl-.

Weak acids are those which completely only partially dissociate when they are dissolved in water to release H+ ions. Majorly neutral molecules exist in solution and not ions.

Eg. CH3COOH is a weak acid since in water, CH3COOH dissociates partially to produce very few H+ and CH3COO- ions.

3. The relation between pH and H+ concentration is:

pH = - log [H+]

Hence with the increase in H+ concentration, pH decreases and solution becomes more acidic and with the decrease in H+ concentration, pH increases and solution becomes more basic.

a. Given,

[H+] = 1.5 x 10-3 M

Hence pH = - log (1.5 x 10-3 )

pH = 3 - log 1.5

pH = 2.82

b. [OH- ]= 3.1 x 10-4 M

Kw = Ionic product of water = [H+]  [OH- ]

At 25oC, Kw = 10-14

Therefore  [H+]  [OH- ] = 10-14

[H+] 3.1 x 10-4 = 10-14

Therefore, [H+] = 10-14 / 3.1 x 10-4

[H+] = 0. 32 x 10-10

pH = - log (0.32 x 10-10)

pH = 10.49

c. When pH = 8.42,

pH = - log [H+]

Hence 8.42 = - log [H+]

Hence [H+] = 3.8 x 10-9 M

d. When pOH = 5.58,

Note: pH + pOH = 14

Hence pH= 14 - 5.58

pH = 8.42

pH = - log [H+]

Hence 8.42 = - log [H+]

Hence [H+] = 3.8 x 10-9 M

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