General Chemistry 2 Math Question

Question

You need a buffer to maintain a pH of 3.20. How many grams of sodium cyanate (NaOCN) would you need to add to 250 mL of a 0.125 M solution of cyanic acid (HOCN, Ka = 3.5 × 10-4 ) to have the desired buffer? Assume no volume change upon the addition of sodium cyanate.

Answer 1

Henderson Hasselbalch:

pH = pKa + log [conjugate base]/[acid]

We know:

pH = 3.20

pKa = -log Ka = 3.46

[acid] [HOCN] = 0.125 M

What we want to find:

[conjugate base] = [NaOCN]

3.20 = 3.46 + log (0.125/x)

solve for x = [NaOCN]

Since you have 250 ml of buffer, you need to convert the above answer to grams.

[NaOCN] x 0.250 L x MW = grams