Answer:
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.
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Remember, for any aqueous solution ...
=> Adding H⁺ => always decreases pH
=> Adding OH⁻ => always increases pH
