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Chem 4 exam question help!!

Hi can anyone help explain how the answers of 075, 1.5 and 1 are derived from this? I mean, REALLY explain it in some simple kind of way if possible? I just cannot seem to get to grips with these at all!!!!!
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Original post by Hexaneandheels
Hi can anyone help explain how the answers of 075, 1.5 and 1 are derived from this? I mean, REALLY explain it in some simple kind of way if possible? I just cannot seem to get to grips with these at all!!!!!


I did a worked example for you, attached here. if you have any further questions, don't hesitate to ask! My teacher taught us to do RICE tables for these questions, ratio, initial, change, equilibrium.

Hope this helps :smile:
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Wow that is so nice of you, thank you so much!! I am self teaching and this was really great help, I have printed this off. Thanks again!!! :smile:

Original post by hannahw807
I did a worked example for you, attached here. if you have any further questions, don't hesitate to ask! My teacher taught us to do RICE tables for these questions, ratio, initial, change, equilibrium.

Hope this helps :smile:
Hi again, I am still working my way through exam paper and have come across another question which has that similar type mathematical theme going but I don't feel I can really apply the RICE formula to it. But I wondered whether you can tell me why, at the end of the answer for it, does it multiply the H+ by 1000/55? I screenshotted the question and the answer. You seem like the person to ask!!! :smile:

Original post by hannahw807
I did a worked example for you, attached here. if you have any further questions, don't hesitate to ask! My teacher taught us to do RICE tables for these questions, ratio, initial, change, equilibrium.

Hope this helps :smile:
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Original post by Hexaneandheels
Hi again, I am still working my way through exam paper and have come across another question which has that similar type mathematical theme going but I don't feel I can really apply the RICE formula to it. But I wondered whether you can tell me why, at the end of the answer for it, does it multiply the H+ by 1000/55? I screenshotted the question and the answer. You seem like the person to ask!!! :smile:


The theme is similar but the reason you cannot use the RICE table here is because RICE is only for equilibria - this is not really an equilibrium question, rather a pH calculation.

I do believe the multiplying by 1000/55 is a different way of expressing dividing by 55/1000. When you divide by a fraction, you flip the fraction to make a multiplication.

I did a worked example for you, which I hope makes it clearer for you :smile:
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I wish I could like this a hundred times! Thanks so very much!!! Are you studying A2 right now as well? You must be well on course for A*
Original post by Hexaneandheels
I wish I could like this a hundred times! Thanks so very much!!! Are you studying A2 right now as well? You must be well on course for A*


I'm studying the Edexcel International A Level, predicted an A (my teacher has little faith in me :colondollar:).

And you? :smile:
Can I ask, what is the difference in doing the international one? I am just interested! And they who have little faith...!! I think you are on for A*.
Well I am external student doing AQA Chemistry, physics and maths and wanting to go to university in Sweden this year or next if I don't get in this time!! :smile:

Original post by hannahw807
I'm studying the Edexcel International A Level, predicted an A (my teacher has little faith in me :colondollar:).

And you? :smile:
Original post by Hexaneandheels
Can I ask, what is the difference in doing the international one? I am just interested! And they who have little faith...!! I think you are on for A*.
Well I am external student doing AQA Chemistry, physics and maths and wanting to go to university in Sweden this year or next if I don't get in this time!! :smile:


For the international syllabus, units 3 and 6, instead of being a practical exam, are both normal written exams. This is because it would be impossible to examine practical assessments all around the world at the same time. And thanks for the vote of confidence xD
Hi again!!

Ok I just attempted another of these evil ones, managed to get the beginning correct (the simple calculate number of moles part) but the rest of this I have totally gone wrong. Grrrr. I have attached a couple pics one of question one of answer. I would really appreciate some of your help on this one!!! :biggrin:

Original post by hannahw807
For the international syllabus, units 3 and 6, instead of being a practical exam, are both normal written exams. This is because it would be impossible to examine practical assessments all around the world at the same time. And thanks for the vote of confidence xD
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Original post by Hexaneandheels
Hi again!!

Ok I just attempted another of these evil ones, managed to get the beginning correct (the simple calculate number of moles part) but the rest of this I have totally gone wrong. Grrrr. I have attached a couple pics one of question one of answer. I would really appreciate some of your help on this one!!! :biggrin:


I kept going wrong on this one too! I ended up bringing out the textbook and found out that in order to get the M3 and 4 you needed to realize that when the concentrations of acid and base are equal, pH = pKa. After that it was easier to figure out what was going on :smile:

Hope this helps!
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So you have really helped me out with this and after a few days obsessivelu learning I feel I am finally to grips with the concept behind it. BUT, I just did a question from a CGP book and my confidence is dashed, but I am CERTAIN they have made a mistake in the answers here. I will upload the pics for you to see the question and the answer. I have underlined where I feel they have made a mistake and I have also uploaded a picture of my working for you to see how I have worked it out. Please tell me they have made a mistake in the book!!!! :biggrin:

Original post by hannahw807
I kept going wrong on this one too! I ended up bringing out the textbook and found out that in order to get the M3 and 4 you needed to realize that when the concentrations of acid and base are equal, pH = pKa. After that it was easier to figure out what was going on :smile:

Hope this helps!
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Original post by Hexaneandheels
So you have really helped me out with this and after a few days obsessivelu learning I feel I am finally to grips with the concept behind it. BUT, I just did a question from a CGP book and my confidence is dashed, but I am CERTAIN they have made a mistake in the answers here. I will upload the pics for you to see the question and the answer. I have underlined where I feel they have made a mistake and I have also uploaded a picture of my working for you to see how I have worked it out. Please tell me they have made a mistake in the book!!!! :biggrin:


I am pretty sure that they made a mistake - you have it right. There doesn't seem to be any way they could get 1.5x10^-3 from what was available. Glad to hear you are getting the hang of this topic! :smile:

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