Dilution and Mixing

Problem 1:  Dilution of Sulfuric Acid

40 mL of 2.5 molar H2SO4 solution will be diluted to 2 L. What is the pH of the diluted solution?

Answer

Before we enter the parameters into the program, please recap: The dilution factor is

    2000 mL / 40 mL  =  50

Hence, the diluted solution contains exactly (2500 mM / 50) = 50 mM H2SO4.

dilu H2SO4

Switch to pure water (click on button New), then click on Reac and enter for “H2SO4” the value 50 mM. Run the calculation with Start.

Instantly, the program will output the result (see right screenshot):

    pH = 1.27

The diluted solution has pH 1.27.

Note. If you solve this task with a pocket calculator you would obtain pH 1.3 as a raw approximation. The deviation from the correct result (pH 1.27) is caused by ionic activity corrections for non-ideal solutions.

Problem 2:  Mixing HCl with NaOH

100 mL of 0.01 molar HCl is mixed with 50 mL of 0.05 molar NaCl. What is the pH of the final solution?

Answer

HCl+Nacl

We begin with pure water (click on button New), then click on Reac and activate the checkbox More Reactions.

Before we enter the parameters, please recap: The final volume is 150 mL (= 100 mL + 50 mL) and, thus, the mixing solution contains:

( 100 mL / 150 mL ) × 10 mM   =   6.67 mM   HCl
(   50 mL / 150 mL ) × 50 mM   =   16.7 mM   NaOH

Run the calculation with Start. The result is shown on the right screenshot. So the answer is: The mixed solution has pH 11.94.

Note. If you solve this task with a pocket calculator you would obtain pH 12. The result calculated by aqion, however, is more accurate. The small deviation is caused by ionic activity corrections for non-ideal solutions.

[last modified: 2015-06-07]