20cm³ of 1mol per dm³ nitrous acid (pKa = 3.34) is added to 40 cm³ of 0.5 mol per dm³ sodium nitrate solution. What is the pH of the solution? Help please because I am unsure as to whether im supposed to calculate the pH using the number of moles or mol per dm³. And can you explain why mole or concentration is used. Thanks
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It appears you a dealing with a buffer solution. However, the mixture is not that of a acid and its conjugate base. Perhaps you meant sodium nitrite (not nitrate). In that case
pH = pKa + log[NO2-]/[HNO2]
[NO2-] = 0.040*0.5/0.060 M (there are 60 cc of solution after mixing)
[HNO2] = 0.020*1/0.060 M
Since the volumes after mixing are the same, you can use moles or concentration, but the formula is based on molar concentration.
[NO2-]/[HNO2] = 0.040*0.5 / 0.020*1 = 1.0; log1.0 = 0, so pH = pKa
pH = pKa + log[NO2-]/[HNO2]
[NO2-] = 0.040*0.5/0.060 M (there are 60 cc of solution after mixing)
[HNO2] = 0.020*1/0.060 M
Since the volumes after mixing are the same, you can use moles or concentration, but the formula is based on molar concentration.
[NO2-]/[HNO2] = 0.040*0.5 / 0.020*1 = 1.0; log1.0 = 0, so pH = pKa