When equal volumes of the indicated solutions are mixed, precipitation should oc

Question

When equal volumes of the indicated solutions are mixed, precipitation should occur only for: A) 2 times 10^-5 M Ag^+ & 2 times 10^-5 M CO_3^2- B) 2 times 10^-5 M Ca^+2 & 2 times 10^-5 M CO_3^-2 C) 2 times 10^-4 M Ca^+2 & 2 times 10^-2 M F D) 2 times 10^-5 M Mg^+2 & 2 times 10^-6 M F E) 2 times 10^-3 M Ba^+2 & 2 times 10^-3 MF a. A buffer was prepared by dissolving 0.10 mol of benzoic acid (K_a = 6.3 x 10^-5) and 0.50 mol of sodium benzoate in sufficient pure water to form a 1.00 L solution. What is the pH of this buffer? pH = __________ b. 70.0 mL of the above buffer is mixed with 2.00 mL of 2.00 M HBr solution. What was the pH of this 72 0 mL solution? pH = ___________

Explanation / Answer

Q1) Oprion C

the ionic product of CaF2 > Ksp so precipitation occurs.

CaF2(s) ------>Ca+2 +2F-

Ksp = [Ca+2][F-]2

Thus Q = 2x10-4 x(2x10-2)2

  = 8.0x10-8

   Q > Ksp (5.3x10-9)

Q2)The Ka of acid = 6.3x10-5 and pKa = 4.20

Give [acid] = 0.1M and [salt] = 0.5M

the pH of buffer is given by Hendersen equation

pH = pKa + log [salt]/[acid]

= 4.2 + log o.5/0.1

= 4.8989

Part c

HA ----------> A- + H+

70x0.1 70x0.5 0 initial mmoles

= 7 35 0

- - 2x2=4 change

11 31 0 after

Now the pH of this buffer is

pH = 4.2 + log 31/11

= 4.6499

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