Chemistry

A 25.00 mL sample of a soft drink was titrated using 12.10 mL of 1.50 M NaOH. Given that the soft drink's monoprotic acid has a Ka value of 2.8 x 10^-5, predict the pH of the soft drink.

I know that the answer is pH 2.85 (our practice questions have an answer key) but I'm really struggling to understand how to do the question. If anyone could help I would be super grateful. Thanks!

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asked by Josh
  1. Call the soft drink, since it is monoprotic, HA.
    Then HA + NaOH ==> NaA + H2O
    mols NaOH = M x L = 0.0121 x 1.5 = ?
    mols HA = the same because of the 1:1 ratio in the balanced equation.
    M HA = mols HA/L HA or M HA = approx 0.7 but yu need a better answer than that estimate. Then

    ........HA ==> H^+ + A^-
    I......0.7M....0.....0
    C......-x......x.....x
    E....0.7-x.....x.....x

    Ka = 2.8E-5 = (x)(x)/(0.7-x)
    Solve for x and convert to pH. I worked through it and 2.85 for pH is right.

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