Question

pH of water and Strong Acid. 30.0mL of TAP H2O and 2.0mL of 1.0M HCl in beaker and pH was measured to be 3.25. Calculate the EXPECTED pH of this solution and find the percent error between the calculated and measured value. What are some possible sources of error?

Answer 1

The no. of mmol of HCl added = 1 mmol/mL * 2 mL = 2 mmol

Now, the concentration of HCl = 2 mmol/(30 + 2) mL = 0.0625 M = [H⁺]_{in HCl}

And [H⁺] _{in water} = 10^-7 M

Now, [H⁺]_total = 0.0625 + 10^-7 ~ 0.0625

Formula: pH = -Log[H⁺]

i.e. pH = -Log(0.0625)

i.e. pH = 1.204

The difference between the calculated and the measured value = 3.25 - 1.204 = 2.046

The percent error = (2.046/3.25)*100 ~ 63%

The possible sources of error:

Tap water (which is not pure) contains soluble minerals, i.e.

1. chorides

2. carbonates

3. sulfates of magnesium and calcium.