Which of the following will decrease the pH of a 50 ml solution of `0.01 M HCI` ?

Options

A. Addition of Mg
B. Addition of 50 ml `0.002 M HCI`
C. Addition of 50 ml `0.01M HCI`
D. Addition of 5 ml 1 M HCI

Answer

D

Explanation

To decreases the pH, we must add more `H^(+)` ions. This can be done by adding more concentrated acid. The addition of Mg (option, I) will increase the pH as it reacts with HCI to decrease the `H^(+)` concentration.
`Mg(s)+2H^(+)(aq.)rarr H_(2)(g)+Mg^(2+)(aq.)`
The addition of `0.002` M HCI (option 2), a less concentrated solution, will increase the pH by decreasing `H^(+)` ion concentration. The addition of `0.01` M HCI (option 3), the same concentrated solultion, does not change the `H^(+)` ion concentration and, hence, the pH.