How would the pH value of an aqueous solution change, when the hydronium ion concentration is increased by a factor of 10?

Select one:
a. decreases by 10
b. decreases by 1
c. increases by 1
d. increases by 10

Increasing H⁺ by 10x => pH decreases by 1 unit

Explanation:

In general, adding H⁺ ions to any aqueous solution ALWAYS causes pH values to fall ( decrease ). Just as adding OH⁻ ions to an aqueous solution causes pH values to rise ( increase ).

Here's a simple calculation demonstrating this...

Given 0.01M HCl(aq) => 0.01M H⁺(aq) + Cl⁻(aq) => pH = -log(0.01) = 2.00

Increase [H⁺] by 10x => 0.10M H⁺(aq) => pH = -log[H⁺] = -log(0.10) = 1.00

Solution with higher H⁺ concentration shows pH decreasing by 1 unit.

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Just to support the above statement about adding OH⁻ ions showing an increase in pH values, the following is also provided FYI ..

Given 0.01M NaOH(aq) => 0.01M OH⁻(aq) + Na⁺(aq) => pOH = -log(0.01) = 2.00 => pH = 14 - pOH = 14 - 2 = 12

Increase [OH⁻] by 10x => 0.10M OH⁻(aq) => pOH = -log[OH⁻] = -log(0.10) = 1.00 => pH = 14 - pOH = 14 - 1 = 13

Increasing [OH⁻] by 10x => increasing pH by 1 unit.

Solution with higher H⁺ concentration shows pH decreasing by 1 unit.

Remember, for any aqueous solution ...

=> Adding H⁺   => always decreases pH

=> Adding OH⁻ => always increases pH