[funzone36]

1. How many moles of NaOH must be added to 0.700 L of 0.175 mol/L HNO2(aq) to obtain a solution with pH=3.3? Assume the temperature is 25°C. (For HNO2, pKa=3.14 at 25°C).

Wrong solution:

3.3 = 3.14 + log [NO2-]/ [HNO2]

10^0.16= 1.4 = [NO2-]/ 0.175
[NO2-]= 0.253 M
moles = 0.253 x 0.700 L = 0.177 = moles NaOH

Anyone know the correct solution?

2.What is the final pH at 25°C when 68.0 mL of 0.133 mol/L NaOH(aq) and 45.0 mL of 0.185 mol/L HA(aq) are mixed. Assume that HA is a weak monoprotic acid with pKa=5.13 at 25°C.

Wrong Solution:
pH = 5.13 + ln (0.133/0.185) = 4.80

Anyone know the correct solution?

[Astrokel]

why do you assume they are buffer questions? they are actually stoichiometry.

[Borek]

why do you assume they are buffer questions? they are actually stoichiometry.

50/50 - a little bit of buffer and a little bit of stoichiometry

3.3 = 3.14 + log [NO2-]/ [HNO2]

10^0.16= 1.4 = [NO2-]/ 0.175

When you add NaOH 0.175 stops to be 0.175 - you have to account for amount of acid neutralized. This is stoichiometry

2.What is the final pH at 25°C when 68.0 mL of 0.133 mol/L NaOH(aq) and 45.0 mL of 0.185 mol/L HA(aq) are mixed. Assume that HA is a weak monoprotic acid with pKa=5.13 at 25°C.

Wrong Solution:
pH = 5.13 + ln (0.133/0.185) = 4.80

Again - think in terms of stoichiometry. How many moles of acid initially? How many moles of strong base initially? How many moles of acid after neutralization? How many moles of conjugated base? Is it buffer, or not?