± pH of a Strong Acid and a Strong Base pH is a logarithmic scale used to indica

Question

± pH of a Strong Acid and a Strong Base
pH is a logarithmic scale used to indicate the hydrogen ion concentration, [H+], of a solution:

pH=log[H+]

Due to the autoionization of water, in any aqueous solution, the hydrogen ion concentration and the hydroxide ion concentration, [OH], are related to each other by the Kw of water:

Kw=[H+][OH]=1.00×1014

where 1.00×1014 is the value at approximately 297 K. Based on this relation, the pH and pOHare also related to each other as

14.00=pH+pOH

The temperature for each solution is carried out at approximately 297 K where Kw=1.00×1014.
Part A
0.15 g of hydrogen chloride (HCl) is dissolved in water to make 7.0 L  of solution. What is the pH of the resulting hydrochloric acid solution?
Express the pH numerically to two decimal places.

pH =

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Part B
0.80 g of sodium hydroxide (NaOH) pellets are dissolved in water to make 7.0L of solution. What is the pH of this solution?
Express the pH numerically to two decimal places.

pH =

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Explanation / Answer

Molecular weight of HCl = 36.46094 g/mol

7 L solution contain .15/ 36.46094 = .00411 mole

1 L solution contain .00411 / 7 = .0005877 mole

so molarity of HCl solution is .0005877 M . Which gives .0005877 M H+ ion.

pH = - log .0005877 =3.231

part B .

Molecular weight of NaOH = 39.997 g/mol

so, 7L solution contain .80 / 39.997 = .02 mole NaOH

       1L solution contain = .02/7 = .00285 mole

molarity of the solution = .00285 M

.00285 molar NaOH will give .00285 molar OH-

pOH =-log .00285 =2.544

we know that

14.00=pH+pOH

so pH = 14 -2.544 = 11.455

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