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# pH calculations watch

1. Easy question.

"The pH of a sample of water is 7.
10 cm^3 of 1.0 mol dm^–3 hydrochloric acid is added to 990 cm^3 of this water.
What is the pH of the solution formed?"

So I will need to look at the total number of moles of H+ per dm^3 . Now HCl will fully dissociate contributing to most H+ ions. However, water will partially dissociate giving also some H+ ions but it is quite small and won't affect the answer significantly. Water will give an additional 10^-7 ions per dm^3 of H+ ions at a specific temperature.
The H+ ions concentration from the HCl = [(10/1000 * 1) / (1000/1000)] = 0.01. Now to add the ones from water, pH = -log(0.01 + 10^-7) = 1.99

So my question, is it a good idea to add the moles of H+ ions from water always? because sometimes they do have an effect, but usually, almost nothing.
2. (Original post by Daniel Atieh)
Easy question.

"The pH of a sample of water is 7.
10 cm^3 of 1.0 mol dm^–3 hydrochloric acid is added to 990 cm^3 of this water.
What is the pH of the solution formed?"

So I will need to look at the total number of moles of H+ per dm^3 . Now HCl will fully dissociate contributing to most H+ ions. However, water will partially dissociate giving also some H+ ions but it is quite small and won't affect the answer significantly. Water will give an additional 10^-7 ions per dm^3 of H+ ions at a specific temperature.
The H+ ions concentration from the HCl = [(10/1000 * 1) / (1000/1000)] = 0.01. Now to add the ones from water, pH = -log(0.01 + 10^-7) = 1.99

So my question, is it a good idea to add the moles of H+ ions from water always? because sometimes they do have an effect, but usually, almost nothing.

The moles of hydrogen from water in this case = 1 x 10-7 = 0.0000001, which cannot significantly affect the 0.01 moles from your calculation.
3. (Original post by charco)

The moles of hydrogen from water in this case = 1 x 10-7 = 0.0000001, which cannot significantly affect the 0.01 moles from your calculation.
Aha. Thank you.

So it's just common sense. When I see a quite small concentration of the acid, then obviously water will have its effect on the calculation now. For example, HCl concentration of 10^-10 mol dm^-3 say.
4. (Original post by Daniel Atieh)
Aha. Thank you.

So it's just common sense. When I see a quite small concentration of the acid, then obviously water will have its effect on the calculation now. For example, HCl concentration of 10^-10 mol dm^-3 say.
yes, basic common sense (forgive the pun)

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Updated: January 18, 2015
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