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# The pH of an HCl solution is 2. Sufficient water is added to make the pH of the new solution is 5.0. The hydrogen ion concentration is reduced:A. ten foldB. sevenfoldC. thousand foldD. hundred fold  Hint: The pH of a solution is the negative logarithm of hydrogen ion concentration. So, substitute the values of hydrogen ion concentration in both cases and compare them to arrive at the answer.

We know the formula for the pH. When we substitute the value of the pH of an HCl solution:
That is, ${\text{pH = 2}}$ so, ${\text{[}}{{\text{H}}^{\text{ + }}}{\text{] = 1}}{{\text{0}}^{{\text{ - 2}}}}$
${\text{pH = 5}}$ So, ${\text{[}}{{\text{H}}^{\text{ + }}}{\text{] = 1}}{{\text{0}}^{{\text{ - 5}}}}$.
If we want to make the pH of 5, then:
${\text{[}}{{\text{H}}^{\text{ + }}}{\text{]}}$ Reduced to$\dfrac{{{\text{1}}{{\text{0}}^{{\text{ - 2}}}}}}{{{\text{1}}{{\text{0}}^{{\text{ - 5}}}}}}{\text{ = 1}}{{\text{0}}^{\text{3}}}$.
That is the hydrogen ion concentration is reduced to option (c) thousand folds.

-We know that in Chemistry, pH is a numeric scale used to specify the acidity or basicity of an aqueous solution. It is approximately the negative of the logarithm to base 10 of the molar concentration, measured in units of moles per litre, of hydrogen ions. More clearly, it is the negative of the logarithm to base 10 of the activity of the hydrogen ion.
-The pH of a neutral solution is 7. The solution with a pH above 7 is basic and pH below 7 is acidic.
-pH can be measured by using a piece of pH test paper, pH indicator sticks or a pH meter.

Note: To solve this problem, you need a basic knowledge about logarithm. Also, you can take the negative logarithm of the pH to get the concentration of hydrogen ions.
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