Question

In: Chemistry

1. Farmers who raise cotton onve used arsenic acid, H3AsO4, as a defoliant at harvest time....

1. Farmers who raise cotton onve used arsenic acid, H3AsO4, as a defoliant at harvest time. Arsenic acid is a polyprotic acid with K₁ = 2.5 * 10^-14, K₂ = 5.6 * 10 ^-8 , and K₃ = 3 * 10 ^-13 .

What is the pH of a 0.500 M sol'n of arsenic acid?

answer ) 1.96

3. A solution is prepared by adding 0.10 mol of KCH₃COO, to 1.00 L of water.

Which statement about the sol'n is correct?

a) the sol'n is basic.

b)the sol'n is neutral

c) the sol'n is acidic

d) the concentrations of K⁺ ions and CH₃COO^- ions will be identical

e) the concetnration of CH₃COO^- ions will be greater than the concetnraion of K⁺ ions.

4. A sol'n is prepared by adding 0.10 mol of KCL, to 1.00 L of water. Which statement about the sol'n is correct?

a) the sol'n is basic

b) the sol'n is neutral

c) the sol'n is acidic

d)one need to know the temperature before any of the above predictions can be made.

e) the values for Ka and Kb for the species in sol'n must be known before a prediction can be made.

I already knew the answers except #2, but I don't know the reason why.

Please explain the questions to me. Thanks!

Solutions

Expert Solution

answer 1

Arsenic is polyprotic which means it can go through differenttitrations. In this problem we are only worried about arsenic acid,this means we use K1. If we were using H2AsO4- then K2 would beused. If HAsO42- then K3 would be used.

answer 2

answer 3: potassium acetate CH3COOK is a salt of starong base (KOH) and weak acid (CH3COOH) ...so the solution will be basic...

CH3COOK + H2O <------> CH3COOH + KOH
CH3COO(-) + K(+) + H2O <------> CH3COOH + K(+) + OH(-)
cancelling common ions from both sides...
CH3COO(-) + H2O <--------> CH3COOH + OH(-)

CH3COOH is a weak acid so it will not dissociate quickly while KOH is astrong base so it will dissociate very quickly...

pH of such solution is given by :
pH = 1/2 [ pKw + pKa + log c ]
Ka for acetic acid is 1.8 X 10^-5
and c is the concentration of the salt solution = 0.1/1 = 0.1 moles/L
so putting the values....
pH = 1/2 [14 + -log (1.8 X 10^-5 + log 0.1 ]
pH = 1/2 [ 14 + 4.745 -1 ] = 1/2 [ 17.745] = 8.873

since pH is above 7 so the solution will be basic...
and answer is (A)

Answer 4:

b) the sol'n is neutral

queen_honey_blossom answered 3 weeks ago

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