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# Calculating molecular formula and stuff including RAM watch

1. Hi,

I've got my Edexcel 360Science C2 structured and multiple choice papers next week and my teacher spends hardly any time exlpaining certain things. Here are the three types of question I am having difficulty with, because I don't have a set method of solving them (kind of like guesswork at the moment).

Do you know the names of these questions so I know what they are? Please show me an easy method and working/explanation if possible, so I can follow!

-1-
---
Chlorine and carbon form a compound with the formula CClx.
If 7.7g of the compound contain 0.6g of carbon what is the value of x?
---

-2-
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There are two isotopes of neon, neon-20 and neon-22.
In a sample of neon there is 91% of neon-20 and 9% of neon-22.
What is the relative atomic mass of neon [sample]?
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-3-
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The equation for the reaction of nitric acid with marble chips is

CaCO3 + 2HNO3 ---> Ca(NO3)2 + H20 + CO2

Calculate the mass of carbon dioxide formed by the reaction of 20g of marble chips with excess nitric acid.
(Relative atomic masses: C = 12, O = 16, Ca = 40)
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Sorry for the long question(s) and thanks a lot for helping me get my good grade in GCSE Chemistry!
2. 1. find the moles of carbon and chlorine;

C 0.6/12 = 0.05 moles , Cl 7.7/35.5 = 0.217

C = 1 atom and Cl = 4 atoms (0.217/0.05 = 4.3)

2. (20 x 91/100) + (22 x 9/100) = 20.18

3. moles of CaCO3 = 20/100 = 0.2 => moles of CO2 = 0.2

moles = mass/RMM so mass = moles x RMM = 8.8g

3. (Original post by EierVonSatan)
1. find the moles of carbon and chlorine;

C 0.6/12 = 0.05 moles , Cl 7.7/35.5 = 0.217

C = 1 atom and Cl = 4 atoms (0.217/0.05 = 4.3)

2. (20 x 91/100) + (22 x 9/100) = 20.18

3. moles of CaCO3 = 20/100 = 0.2 => moles of CO2 = 0.2

moles = mass/RMM so mass = moles x RMM = 8.8g

Thank you very much! I am impressed.

1. Isn't it 7.1/35.5 (removing the mass of carbon from the compound)? Which should result in the answer being 4, which is correct.

2. So that gives you the ratio of each neon isotopes... so I would do the same if the question gave weight or mass of the elements (e.g. tonnes, grams)?

3. So it's 0.2 (moles of CaCO3) multiplied by the RMM of CO2 (44) to give 8.8g. I like it.

Thanks again, I will find some questions and practice. Does anyone have any like these?
1. Isn't it 7.1/35.5 (removing the mass of carbon from the compound)? Which should result in the answer being 4, which is correct.
Sorry I misread the question, so yes 7.1/35.5

2. So that gives you the ratio of each neon isotopes... so I would do the same if the question gave weight or mass of the elements (e.g. tonnes, grams)?
No, you have to convert it to moles first

3. So it's 0.2 (moles of CaCO3) multiplied by the RMM of CO2 (44) to give 8.8g. I like it.
Yeah, because for every molecule of CaCO3 you have you get one molecule of CO2 out (1:1 ratio)

Thanks again
welcome

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