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# Determining unknown quantities question watch

1. Anyone know the answer to this? It was in one of the multiple choice tests we were given online. I got the answer wrong and was given the correct answer (3) but not any of the working out so I don't know how to reach the answer.

Read the information below about an acid represented by the formula HxY.

•One mole of the acid represented by HxY ionises to produce x mol of hydrogen ions.
•The molar mass of the acid is 192 g mol−1.
•2.4 g of the acid was dissolved in water and the volume made up to 250 cm3.
•20 cm3 of this solution was neutralised by exactly 25.0 cm3 of a 0.12 mol dm−3 solution of NaOH(aq).

What is the value of x?

Thanks
2. Hi,

Since the Mr is 192g, you can find the number of moles in your 250 cm^3 solution by doing 2.4/192. This gives you 0.0125 moles.

Since you are only using 20cm^3 of your solution, you multiply 0.0125 by 20/250 to find the number of moles present in your 20cm^3 solution. This comes to 1 x 10^-3 moles

Then workout the no. moles of NaOH with vol x conc, which gives you 3 x 10^-3 moles. You then have a molar ratio of HxY: NaOH, 1:3.

Therefore your reaction must be HxY + 3NaOH --> 3NaY + 3H20. Hence, your x value must be 3.

Hope this helps
3. Thanks, that's great! I knew I was missing a step, turns out I forgot to multiply to find the amount of moles in 20cm^3 rather than 250. Thanks again

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