Calculate the total concentration of hydronium ion (H_3O^+)in aqueous solution i

Question

Calculate the total concentration of hydronium ion (H_3O^+)in aqueous solution if 0.0080 moles of hydrocyanic acid (HCN, Ka = 4.9 times 10^-10) are added to a 500 mL solution with an existing pH of 4.05 at 25 degree C. You may assume the addition of HCN does not affect the total volume of the solution. b) Consider an aqueous solution prepared by adding 0.0080 moles of HCN to 500 mL of pure water at 25 degree C. Would the HCN solution in part (a) have a higher or lower percent dissociation than this solution? Justify your reasoning. Your first attempt should be done without the use of calculations.

Explanation / Answer

3. a) [H+] already present=(10^-4.5)/2

Contribution of HCN=(Ka*c)^1/2=(0.0080*4.9*10^-10)^1/2=1.97*10^-6

Thus total [H+]=5*10^-5.5+1.97*10^-6=1.777*10^-5

b) The HCN solution in part a) will have lower percent dissociation than this solution as in solution a) already an acid is present and so due to common ion effect the second acid will dissociate less amount.

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