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# Chemistry help watch

1. Hello I'm having a hard time working out how to answer this question and would like some help:
An acid, H2X, reacts with sodium hydroxide as shown in the equation below: H2X(aq) + 2NaOH(aq) → Na2X(aq) + 2H2O(l)
A solution of this acid was prepared by dissolving 1.92 g of H2X in water and making the volume up to 250 cm3 in a volumetric flask.
A 25.0 cm3 sample of this solution required 21.70 cm3 of 0.150 mol dm–3 aqueous NaOH for complete reaction.

a )Calculate the number of moles of NaOH in 21.70 cm3 of 0.150 mol dm–3 aqueous NaOH. For this question to find the moles would I do 21.70x0.150 and then divide the answer by 2?

b )Calculate the number of moles of H2X which reacted with this amount of NaOH. Hence, deduce the number of moles of H2X in the 1.92 g sample.

c )Calculate the Mr of H2X
2. (Original post by Yip9090)
Hello I'm having a hard time working out how to answer this question and would like some help:
An acid, H2X, reacts with sodium hydroxide as shown in the equation below: H2X(aq) + 2NaOH(aq) → Na2X(aq) + 2H2O(l)
A solution of this acid was prepared by dissolving 1.92 g of H2X in water and making the volume up to 250 cm3 in a volumetric flask.
A 25.0 cm3 sample of this solution required 21.70 cm3 of 0.150 mol dm–3 aqueous NaOH for complete reaction.

a )Calculate the number of moles of NaOH in 21.70 cm3 of 0.150 mol dm–3 aqueous NaOH. For this question to find the moles would I do 21.70x0.150 and then divide the answer by 2?

b )Calculate the number of moles of H2X which reacted with this amount of NaOH. Hence, deduce the number of moles of H2X in the 1.92 g sample.

c )Calculate the Mr of H2X
For a) What is the equation linking moles, concentration and volume? Make sure that all of your units are the same so if the concentration is in moles dm-3, the volume should be in cm3.

b) Do you know how to use a balanced symbol equation to find the mole ratio between different reactants? From this, you need to note that this is the amount of H2X in 25cm3 of solution and we want the amount of H2X in the original sample.

c) This is basically using the stuff you've calculated up to now to finish the question off
3. (Original post by ecila21)
For a) What is the equation linking moles, concentration and volume? Make sure that all of your units are the same so if the concentration is in moles dm-3, the volume should be in cm3.

b) Do you know how to use a balanced symbol equation to find the mole ratio between different reactants? From this, you need to note that this is the amount of H2X in 25cm3 of solution and we want the amount of H2X in the original sample.

c) This is basically using the stuff you've calculated up to now to finish the question off
The question for A was "Calculate the number of moles of NaOH in 21.70 cm3 of 0.150 mol dm–3 aqueous NaOH" Do I not need to use the formula of Concentration x volume to solve this? Or is it something else?

And for B I am not fully sure on how to use it to find tge mole ratio
4. (Original post by Yip9090)
The question for A was "Calculate the number of moles of NaOH in 21.70 cm3 of 0.150 mol dm–3 aqueous NaOH" Do I not need to use the formula of Concentration x volume to solve this? Or is it something else?

And for B I am not fully sure on how to use it to find tge mole ratio
For A, yes but you need to make sure that the units are correctly converted. You did 21.7 x 0.15 but you have one unit in cm3 and one in mol dm-3 so you need to make sure that they are both in a unit relating to dm3, if that makes sense.

Okay, so you've been given the balanced symbol equation. The number in front of each compound shows how many moles are reacted compared to other compounds. So H2X + 2NaOH means that for every mole of H2X that reacts, 2 moles of NaOH react. So if you calculate that there are 8 moles of NaOH, for example, there will be 4 moles of H2X as the ratio of H2X : NaOH is 1:2
5. (Original post by ecila21)
For A, yes but you need to make sure that the units are correctly converted. You did 21.7 x 0.15 but you have one unit in cm3 and one in mol dm-3 so you need to make sure that they are both in a unit relating to dm3, if that makes sense.

Okay, so you've been given the balanced symbol equation. The number in front of each compound shows how many moles are reacted compared to other compounds. So H2X + 2NaOH means that for every mole of H2X that reacts, 2 moles of NaOH react. So if you calculate that there are 8 moles of NaOH, for example, there will be 4 moles of H2X as the ratio of H2X : NaOH is 1:2
Ah okay, I get what you are saying for A and will need to convert it to make it the same units.

I also see where you are going for question B but I am still unsure what the 1.92 g plays into.
6. (Original post by Yip9090)
Ah okay, I get what you are saying for A and will need to convert it to make it the same units.

I also see where you are going for question B but I am still unsure what the 1.92 g plays into.
Yes that's exactly it so you were along the right lines already

So in the question, it says that 1.92g of solid H2X is dissolved in 250 cm3 of water. The sample is only 25 cm3, meaning there is 0.192g of H2X in the reaction. Weve found the moles of NaOH, which we can use to find the moles of H2X in the reaction but we need to find the total number of moles of H2X that we started with to help us find X
7. (Original post by ecila21)
Yes that's exactly it so you were along the right lines already

So in the question, it says that 1.92g of solid H2X is dissolved in 250 cm3 of water. The sample is only 25 cm3, meaning there is 0.192g of H2X in the reaction. Weve found the moles of NaOH, which we can use to find the moles of H2X in the reaction but we need to find the total number of moles of H2X that we started with to help us find X
Ah I see how to do this question now, thanks for the help.

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Updated: November 7, 2017
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