Question

Here are some questions and their answers. I would appreciate it if someone could explain to me step by step how to get the answer?

1) A student pipets 25 mL of propanoic acid from a stock container labeled 0.18 M propanoic acid. The student then dilutes the 25 mL to 100 mL with distilled water. He labels the bottle of 100 mL of propanoic acid with what concentration? The answer for this is 0.045M propanoic acid, but can someone show step by step how to get to this answer?

2) Which of the following three salts would be acidic if dissolved in water: I. KCN II. NH4Br III. C2H5NH3Cl
answer: II and III only (how and why???)

3) What is the OH− concentration in Drano, a common drain cleaner that has a pH of 12.3 at 25oC? Answer: 0.02 M

4) What is the hydronium ion concentration (x) in a 1.23M solution of hypochlorous acid (HOCl) at 25oC. Answer: sqrt 1.23 ∗ 3.5x10^-8

5) The two acid dissociation constants for sulfurous acid, H2SO3, are 1.3× 10–2 and 6.3× 10–8 at 25oC. The base constant, Kb, for HSO3– is:7.7 × 10^–13

Answer 1

1)

we know that

for dilution

moles of solute = constant

also

moles = molarity x volume

so

M1V1 = M2V2

0.18 x 25 = M2 x 100

M2 = 0.045

so

the final concentration is 0.045 M

2)

for a salt to be acidic

it should be formed from strong acid and weak base

KCN fromed from weak acid (HCN) and strong base (KOH)

NH4Br ---> strong acid (HBr) and weak base (NH3)

C2H5NH3Cl ---> strong acid (HCl) and weak base ( C2H5NH2)

so

the answer is ll and lll

3)

we know that

pH = -log [H+]

so

12.3 = -log [ H+]

[H+] = 5 x 10-13

now

[H+] [OH-] = 10-14

so

5 x 10-13 x [OH-] = 10-14

[OH-] = 0.02 M

4) given HOCl

it is a weak acid

for weak acids

[H+] = sqrt ( Ka x C)

aslo

Ka for HOCl is 3.5 x 10-8

so

[H+] = sqrt ( 1.23 x 3.5 x 10-8)

[H+] = 2.075 x 10-4

5)

we know that

Ka x Kb = 10-14

we need Kb for HS03-

so

Ka should be for its conjugate acid H2S03

so

in tis case

Ka = Ka1 = 1.3 x 10-2

so

1.3 x 10-2 x Kb = 10-14

Kb = 7.69 x 10-13