Charles CollingwoodChemistry and Integrated Science Teacher

AQA chem 4 Jan 12 - Question 4a

Name: AQA chem 4 Jan 12 - Question 4a
Uploaded: 2025-11-19T18:50:49.013690
Duration: 2 min 37 s
Channel: Charles Collingwood
Description: Short tutorial video (Charles Collingwood) solving an AQA chemistry question: dilution of 25.0 cm^3 of 0.0850 M HCl by adding 100 cm^3 water, computing…

19 Nov 2025 · 3 min

Watch on YouTube

2:37

Short tutorial video (Charles Collingwood) solving an AQA chemistry question: dilution of 25.0 cm^3 of 0.0850 M HCl by adding 100 cm^3 water, computing moles, new concentration after dilution, and resulting pH. The speaker computes moles = 0.002125 mol, new total volume = 0.125 dm^3, new concentration = 0.0170 M, and pH ≈ 1.77 (to 2 d.p.).

Questions this video answers

25.0 cm^3 of 0.0850 M HCl.

0.002125 mol (0.025 × 0.085).

125 cm^3 (0.125 dm^3).

0.0170 mol dm^-3 (0.002125 ÷ 0.125).

pH = 1.77 (−log10[H+] with [H+] = 0.0170 M).

Chapters

Okay, we're asked to find the new PH of a solution. We start off with 25cm cubed of 0.0850. So we have to calculate the moles of hydrochloric acid. First of all, moles of HCL. That's going to equal 0.025 the number of liters multiplied by 0.085 the concentration. We multiply that out 0.025 times 0.085, that comes to 0.002125 moles. So this is the number of moles of hydrochloric acid we've got. Okay, fine. Then the new volume we're adding 100 centimeter cubed distilled water. So now the new volume is 125 or 0.125 DM cubed liters. So the new concentration concentration equals moles over volume. Moles per liter. The number of moles we have there, 0.002125 still in the calculator. Divide it by 0.125 and that gives us 0.017 moles per liter. The molarity. Okay, the PH of the new solution formed. Well, ph. The definition of ph, ph is equal to minus log 10. Hydrogen ion concentration. Oh, what happened there? Minus log log 10. We're not having much luck here. Log hydrogen ion concentration. There we go. So the hydrogen ion concentration, well, it's hydrochloric acid. So it's a strong acid. So it's got the same hydrogen ion concentration, the molarity of the hydrochloric acid. So we take the log of 0.017. The log is minus 1.769, minus 1.77 and so the pH is plus 1.77. Give the answers to two decimal places. It's actually 1.76955, so that's 1.77.