# Video: Calculating the pH of a Solution Formed from Dissolving the Same Amount of HCl and NaOH in a Known Volume of Water

What is the pH of a solution prepared by dissolving 1.0 × 10⁻³ mol of HCl and 1.0 × 10⁻³ mol of NaOH in enough water to yield 1.0 L of solution? [A] Zero [B] Between 0 and 7 [C] 7 [D] Between 7 and 10 [E] Greater than 10

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### Video Transcript

What is the pH of a solution prepared by dissolving 1.0 times 10 to the minus three moles of HCl and 1.0 times 10 to the minus three moles of NaOH in enough water to yield 1.0 liter of solution? A) Zero. B) Between zero and seven. C) Seven. D) Between seven and 10. Or E) Greater than 10.

The pH is defined as the negative log of the concentration of hydrogen ions in the solution. A pH of seven corresponds to a solution that’s neutral, while a pH greater than seven is basic. And a pH that’s less than seven is acidic. In this question, we’re dissolving HCl and NaOH together in a solution. HCl, which is called hydrochloric acid, is an acid, and NaOH, which is called sodium hydroxide, is a base. So when we dissolve these two chemical species in a solution, they would react to form water and a salt, NaCl.

The question tells us that we have 1.0 times 10 to the minus three moles of hydrochloric acid and 1.0 times 10 to the minus three moles of sodium hydroxide. According to our balance chemical equation, every one mole of hydrochloric acid reacts with one mole of sodium hydroxide. Since the amount of sodium hydroxide and the amount of hydrochloric acid is the same and they react one to one, these two will react completely, which means that when the reaction is finished, both of them will be completely used up.

So when the reaction is finished, only water and sodium chloride will be remaining in the solution. We’re looking for the pH of our solution. Since there will be no more hydrochloric acid or sodium hydroxide after they react, we really want the pH of our products. Water is neutral, and sodium chloride is a neutral salt. So both of our products are neutral. A neutral pH is a pH of seven, which matches answer choice C. So the pH of our solution of hydrochloric acid and sodium hydroxide is seven.