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# A2 Chemistry - calculations watch

1. "A student analysed two unknown chemicals. From the results of each experiment, the formula of each compound can be determined.

A student dissolved 0.270g of an anhydrous compound of iron and chlorine in water. The student then added an excess of aqueous silver nitrate to precipitate aqueous chloride ions as solid silver chloride. The student collected 0.610g of AgCl(s)

Calculate the formula of the iron compound."

The way i did this was is basically we have 0.270g of iron and chlorine and then we get 0.610g of AgCl so we could say that 0.610 - 0.270 = 0.34g of Ag??? from there i figured out the moles of Ag----> 0.34g/108 = 0.00314 moles. Then to get the mass of Cl, it would be 0.61-0.34 = 0.27g of Cl----> moles = 0.27g/35.5 = 0.0076

0.00314:0.0076 = 1:2 ratio that means there are 2 moles of Cl

Which makes the formula for iron FeCl2. Is that the right way of doing it and is the answer right?

Thanks in advance.
2. You need to work out the number of moles of AgCl first (you know the mass and Mr), you can't subtract the masses like that. That'll be equal to the number of moles of Cl, work out how much that'd weigh and take that away from the original mass.

You're left with the mass of iron, find the number of moles and you'll get a whole number ratio with that of Cl.

I got the same answer as you so I guess you can do it that way :P. Is that how it's been taught to you?
3. (Original post by Preasure)
You need to work out the number of moles of AgCl first (you know the mass and Mr), you can't subtract the masses like that. That'll be equal to the number of moles of Cl, work out how much that'd weigh and take that away from the original mass.

You're left with the mass of iron, find the number of moles and you'll get a whole number ratio with that of Cl.

I got the same answer as you so I guess you can do it that way :P. Is that how it's been taught to you?
Nope, taught myself that oh so that works then..... niceee.
4. But i will learn the other way as well cause it's similar in terms of method but probably how the examiners want me learn it which i need in order to ace this exam. So thanks mate for showing me that method!!!
5. If it's self-taught you might be better off using the molarity method.

For one thing your method gets a ratio of 1 : 2.4, mine gets 1 : 1.98. Using masses like that might work in some simple cases but it's not accurate and reliable - I was always taught to only work like that in moles.

Ah, thanks, no problem!
6. I dont follow your method at all, but i got the same answer by finding the mass of Cl in AgCl and then comparing that with mass of iron chloride...
7. (Original post by boromir9111)
"A student analysed two unknown chemicals. From the results of each experiment, the formula of each compound can be determined.

A student dissolved 0.270g of an anhydrous compound of iron and chlorine in water. The student then added an excess of aqueous silver nitrate to precipitate aqueous chloride ions as solid silver chloride. The student collected 0.610g of AgCl(s)

Calculate the formula of the iron compound."

The way i did this was is basically we have 0.270g of iron and chlorine and then we get 0.610g of AgCl so we could say that 0.610 - 0.270 = 0.34g of Ag??? from there i figured out the moles of Ag----> 0.34g/108 = 0.00314 moles. Then to get the mass of Cl, it would be 0.61-0.34 = 0.27g of Cl----> moles = 0.27g/35.5 = 0.0076

0.00314:0.0076 = 1:2 ratio that means there are 2 moles of Cl

Which makes the formula for iron FeCl2. Is that the right way of doing it and is the answer right?

Thanks in advance.
You are subtracting the mass of iron and chlorine from silver and chlorine which would give you the mass of silver minus iron NOT silver alone.

AgCl - FeClx = Ag - Fe

However what you could do is to say that AgCl is composed of 108/143.5 parts of silver therefore mass of silver = 0.610 x 109/143.5 = 0.459 g and the mass of chlorine = 0.61 x 35.5/143.5 = 0.151 g

Subtract this mass from that of FeClx to gove mass of iron = 0.270 - 0.151 = 0.119

now you have mass ratios you turn them into moles ratios by dividing by the RAMs

Fe: 0.119/56 Cl: 0.151/35.5
Fe: 0.119/56 Cl: 0.151/35.5
Fe: 0.0021 Cl: 0.0042

Therefore FeCl2
8. (Original post by charco)
You are subtracting the mass of iron and chlorine from silver and chlorine which would give you the mass of silver minus iron NOT silver alone.

AgCl - FeClx = Ag - Fe

However what you could do is to say that AgCl is composed of 108/143.5 parts of silver therefore mass of silver = 0.610 x 109/143.5 = 0.459 g and the mass of chlorine = 0.61 x 35.5/143.5 = 0.151 g

Subtract this mass from that of FeClx to gove mass of iron = 0.270 - 0.151 = 0.119

now you have mass ratios you turn them into moles ratios by dividing by the RAMs

Fe: 0.119/56 Cl: 0.151/35.5
Fe: 0.119/56 Cl: 0.151/35.5
Fe: 0.0021 Cl: 0.0042

Therefore FeCl2
This makes much more sense as mentioned in another post as my method of wanting to figure is the same but the chemistry behind it is not right even though i get the right answer i would get no method marks. This is way is much simpler than my method lol, Thanks mate!!!!

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