Gravimetric Analysis trouble (1 Viewer)

[Aerlinn]

Two evil gravimetric analysis questions I have trouble doing:

A 2.200g sample of an unknown organic compound was extracted from plant material. When burnt in oxygen, hydrogen in the compound was converted to 1.32g of H2O, carbon to 3.23g of CO2 and the remainder of the coupound was oxygen.
a) Find the empirical formula of the compound
b) A further sample was analysed by mass spectrometry and found to have a molar mass of 60.0g mol (-1 as superscript). What is the molecular formula of the compound?

Some shavings of a magnet known to be made of a nickel alloy were dissolved in nitric acid. Dimethylglyoxime solution was added to produce bright red precipitate of nickel dimethylglyoxime, Ni(C2H7O2N2)2. The precipitate was then filtered and collected using a Gooch crucible, washed and dried in an oven at 110C.
Experimental data:
mass of alloy = 0.102g
mass of crucible= 23.515g
Mass of crucible + precipitate:
1st weighing 24.996g
2nd weighing 23.651g
3rd weighing 23.650g
Calculate the percentage by mass of nickel in the alloy.

 

[xiao1985]

Q1) a)

1.32g of H2O means 1.32/18 = 0.07333moles of H2O = 0.146666 moles of H
= 0.14666 g of H
3.23 g of CO2 = 3.23 / (12+16x2) = 0.07341 moles of CO2 = 0.074341 moles of C = 0.881g of C
2.200-0.14666-0.881=1.172431g of O = 0.073277 moles of O (note, O, not O₂)

Hence, they are in the mole ratio of C: H : O = 0.074341 : 0.14666:0.073277 roughly = 1:1:2
therefore, imp formula would be CH₂O

b)

MW = 60.0 g mol^-1
imp formula = C_xH_2xO_x
MW = (12 +2 + 16)x = 30x

therefore x = 2
therefore formula = C₂H₄O₂


Q2

mass of ppt = 3rd weight - mass crucible (because 1st and 2nd weight contains traces of water and hence are not usable)
= 23.650 - 23.515 = 0.135g of nickel dimethylgloxime
MW of nickel (mumble jumble) = 58.7 + 12.01 x 4 + 14 x 1.008 + 16 x 4 + 14.01 x 4 = 240.892 g mol^-1

moles of nickel mumble jumble = 0.135 / 240.892 = 5.60417 E -4 moles
moles of nicke present = 5.60417 E -4 moles
mass of nickel present = 5.60417 E -4 x 58.7 = 0.0329g

precent of nickel by mass: 0.0329 / 0.102 x 100% = 32.25%

 

[Aerlinn]

Thanks

I'm doing this question:
There is concern in the community about water pollution that is a result of phosphates being added to washing powders to improve their cleaning ability. The phosporus in a 2.0g sample of washing powder is precipitating as Mg2P2O7. The precipitate weighs 0.085g.
What is the %, by mass, of phosphorus in the washing powder?

In the working out my teacher's done for this question, she found the no. of mole of Mg2P2O7 first, which was 4.28*10^-4. And then she's found the no. mole of P by going 2 * the no. mole of Mg2P2O7, before finding the mass of P.

Why did she times the no. mole by 2? Can't make sense of it...

 

[xiao1985]

because one mole of Mg2P2O7 will be equivalent of 2 moles of Phosphate (PO4 3-)

 

[Aerlinn]

Ok, can you elaborate as to... how you know that?

 

[xiao1985]

P2O7 gets turned into PO4 somehow yes?!

and since ALL P2O7 gets turned into PO4, and there's no other place phosphorus can go, hence for every mole of P2O7, it produces 2 moles of PO4... (somehow, it grabs extra oxygen from somewhere...

and hence for everymole of Mg2P2O7, it means there are 2 moles of PO4 in the start...

oh btw, you need to know phosphate has formula of PO4 3-

 

[Aerlinn]

Ah, I knew that ^_^ I was just... very slow and not thinking properly...