HSC question help (1 Viewer)

[abdog]

A student carried out an investigation to analyse the sulphate content of lawn fertiliser. The student weighed out 1g of fertiliser and dissolved it in water. 50ml of 0.25mol/l barium chloride solution was then added. A white precipitate of barium sulphate formed, which weighed 1.8g.

Calculate the percentage by mass of sulphate in the fertiliser. HELP!

 

[madharris]

If you're only up to the acidic environment then you shouldn't be doing this however ill post the answer anyway

moles of BaSO4
n= m/M
n= 1.8/233.4
n=7.7…x10^-3 mol
.: moles of sulcate = 7.7…x10^-3mol

mass of sulfate
m= nxM
m= 7.7…x10^-3 x 96.1
m=0.741131105g

% = 0.7411... x 100 = 74% (2 sig fig)

not sure if that's right... i think i should stop posting on the chem forum... have absolutely no idea

 

[abdog]

If you're only up to the acidic environment then you shouldn't be doing this however ill post the answer anyway

moles of BaSO4
n= m/M
n= 1.8/233.4
n=7.7…x10^-3 mol
.: moles of sulcate = 7.7…x10^-3mol

Why is the moles of sulphate the same as the moles of BaSO4? Where about Ba? We are only at the beginning of Acidic environment, but my teachers are slow so I have to move quick.

 

[nightweaver066]

In BaSO4, the number of moles of Ba2+ are the same as the number of moles as SO42- as they react in a 1:1 ratio.

Since Ba2+ + SO42- -> BaSO4, the number of moles of Ba2+ = the number of moles of SO42- = the number of moles of BaSO4

 

[lochnessmonsta]

Im pretty sure madharris is right.

the formula weight of BaSO4 is 137.3 + 32.07 + 4x16 = 233.37

The moles of sulfate are the same as in a chemical equation the moles are in equal whole number ratios (at least for most of them)

That means that ===> Ba^2+ + SO4^2- <> BaSO4(s)

As there is .0077 moles of BaSO4 in the equation, there is .0077 moles of sulfate in the equation

 

[abdog]

I see. So if it was H2SO4, then the moles of Hydrogen must be multiplied by 2? Is that right?

 

[nightweaver066]

I see. So if it was H2SO4, then the moles of Hydrogen must be multiplied by 2? Is that right?

Yep. To be clear, it is 2 moles of H, not 2 moles of H2.

 

[lochnessmonsta]

What do you mean by if it was H2SO4? That is an acid an there is no mention of an acid in the question

 

[abdog]

Thank you so much!

 

[abdog]

It was just an example to see how the moles thing work. Anyway, thank you for your input as well.