Olly_s15's Chemistry Question Thread

[olly_s15]

Hey guys, bit stuck on this one 

An impure sample of iron(II) sulfate, weighing 1.545g, was treated to produce a precipitate of . If the mass of the dried precipitate was 0.315g, calculate the percentage of iron in the sample.

[Martoman]

We know that
Fe(2+) + (whatever) >> Fe203.

We can do two things here. We can either balance the equation like:

2Fe(2+) + (whatever) >>Fe203

Then 1 mole of the product makes 2 mol of Fe(2+).

OR as I like to do it: we know from the actual equation, Fe203 that 1 mol of Fe203 will contain 2 mol of Fe.


Thus we know that mol(Fe203) = .315/(55.9*2+48)
                                          = .00197 mol.

Then mol(Fe) = 2 * mol (Fe203)
                   =  .0039mol.

Then mass(Fe) = .0039 * 55.9
                     = .22 g

Then from the original mass, we can work out a percentage.

.22/1.545 *100 = 14.26 %

[olly_s15]

I see.. Thanks

[Martoman]

happy to help.

[olly_s15]

A 2.203g sample of an organic compound was extracted from a plant. When it was burned in oxygen, the hydrogen in the compound was converted to 1.32g of water and the carbon was oxidised to 3.23g of carbon dioxide. Find the empirical formula of the compound.

Just having trouble where to start.

[superflya]

write ur equation

find mol of and

n(H)= and n(c)=

find their masses using

then find mass of oxygen by subtracting the masses of hydrogen and carbon that u just found from the mass of the compound (2.203g)

u can then find the moles of oxygen.

u shood be able to finish it off from there.

[longy1991]

I'm just wondering here, but does anyone know what the treatment would be for the iron ions? coz it seems that they have changed from iron (II) to iron (III) ions, as in rust, but i'm unsure of what has happened here.

[olly_s15]

Just need help in determining the mole ratios with this question:

25mL of excess 0.75M Ba(OH)2 was added to 25mL of a solution of H2SO3 of unknown concentration. It then required a titre of 12.95 mL of 0.405M HClO4 to neutralise the excess. What was the concentration of the H2SO3 solution?

[fady_22]

1st equation:


2nd equation:


Then the normal back titration calculations are required to answer the question.

[superflya]

u can find by using they concentration and volume are both given in the first part of the question.

u can then find using once again the concentration and volume are given in the question.

2 mol reacts with 1 mol so equals

then find the that reacted with by subtracting the amount of from the original amount of

u can then find

[olly_s15]

Okay, this is very simple but I don't have answers to these questions so I just wanna be 100% I'm doing this correctly:

For 20mL of 0.1M HCl, what is the pH?

[superflya]

find moles of HCl using n=cv and the moles of H will equal the moles of HCl as 1 mole of H is present in 1 mole HCl

edit: forget what i just said, completely unessecary, ur given the concentration. \facepalms

[fady_22]

You have the concentration of HCl, thus you have the [H+], which is 0.1 M.
pH=-log[H+]=-log[0.1]= 1

The pH is 1.

[Twenty10]

-log10[H+]
so -log10[.1]     also same as    -log10(10)^-1
   so = 1

hope its wat u got!

[olly_s15]

Yeah all good, just had a whole heap of these questions without an example or answers so I wasn't completely sure I was doing it correct. Thanks for confirming