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LoginChemistry - Mole concept (check)
3 posts
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1. 3.2g of sivler nitrate was added to 50cm3 of 0.4mol/dm3 of calcium chloride. calculate the mass of silver chloride precipitated at the end of experiment.
The equation of reaction: 2AgNO3 + CaCl2 -> 2 AgCl + Ca(NO3)2
Mr of AgNO3 = 170
No of moles of AgNO3 = 3.2/170 = 0.01882352941 moles
50cm3 of CaCl2 = 0.05dm3
Concentration in mol/dm3 = no of moles of CaCl2 / vol. of soln.
0.4 = no of moles of CaCl2 / 0.05
.: no of moles = 0.4 *0.05 = 0.02 moles
Limiting factor is silver nitrate.
Compare silver nitrate with silver chloride. (in terms of mole ratio)
no of moles needed for Agcl = 0.1882352941 moles
Find Mr of AgCl = 143.5
Mass of AgCl: 143.5 *0.018823352941
= 2.70g
2. Aqueous barium chloride reacts with sulphuric acid according to equation below
BaCl2 + H2SO4 -> BaSO4 + 2HCl
a. calculate the mass of baiurm sulphate precipitate produced when 250cm3 of 0.2mol/dm3 of sulphuric acid reacts with 200cm3 of 0.25mol/dm3 of barium chloride.
Answer's too lengthy and I don't have much time to spare... so i'll just show part of the answer.
In this question, there's no limiting factor as no. of moles for the reactants are exactly the same.
The answer I got is 11.65g
3rd question.
zinc reacts with silver ions according to the equation : Zn + 2Ag+ ->Zn2+ + 2 Ag.
calculate mass of zincions formed when 0.4mol of zinc atoms are added to 5cm3 of 0.6mol/dm3 of silver ions.
limiting factor is Ag+ for this case.
Mole ratio for Ag+ : Zn2+
2 : 1
= 0.009 : 0.0045
Last part: is Mr of Zn2+ same as zinc atoms 65? if thats the case, then
0.0045 = mass of zn2+ / 65
mass = 65 *0.0045
=2.925g
Help me check the following questions. but they should be correct, as far as i know. find limiting factor for each case.
1. calcium hydroxide : 7.5g/dm3 and 20 cm3
Nitric acid : 0.81mol/dm3 and 25cm3
limiting factor is calcium hydroxide (ans)
2. Potassium hydroxide: 12g/dm3 and 32cm3
Ammomium sulphate - 2.9g
limiting factor is potassium hydroxide(ans)
3. calcium metal - 6g
sulphuric acid : 0.8mol in 500cm3
calcium has 0.15moles and suplhuric acid 0.8moles.
limiting factor is calcium. (ans)
for the last 3 questions, please try to work out the statements on your own because i'm in a rush for time. sorry... and thanks!
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Moderator'A' & 'O' Levels Chemistry & Biology Ex-MOE Teacher Tutor
840 posts since May '05-
If you're confident you know what you need to do for each of these questions (all of which are straightforward, nothing tricky), then the only reason why you might end up with the wrong answer, is due to careless mistakes.
Just to say (even though your post wasn't addressed to me, it was for everyone), I'm personally not inclined to work out all the questions to check your answers for you (or anyone else, for that matter). I'll only give advice and suggestions when asked for specific guidance or assistance on specific doubts, queries, concepts or topics.
However (don't get me wrong), it's still perfectly ok for you to make such posts here. I'm sure there are many students of your level (eg. 'O' or 'A') who would like the practice, and they can double check your answers for you.
Let's await replies from these other students, if any.
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