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# Need help with moles calculation question! watch

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1. Heya! I'm a bit confused about a question on my homework and was wondering if anyone can help me out. I've put my working out in red below each question. If someone could see if my answers are correct and help me out with the last one especially, that would be much appreciated! Thanks!

1) A tank contained 400m3 of waste hydrochloric acid. It was decided to neutralise the acid by adding slaked lime, Ca(OH)2.

a) The concentration of the acid was first determined by titration of a 25cm3 sample against 0.200mol.dm-3 sodium hydroxide, of which 31.3cm3 were required.

i) Calculate the concentration of the hydrochloric acid in the sample in mol.dm-3 giving your answer to 3 significant figures.

mol = 0.0313 x 0.200 = 0.00626 mol of NaOH

1 NaOH : 1 HCl so 0.00626 mol NaOH : 0.00626 mol HCl

concentration = 0.00626 / 0.025 = 0.2504 mol dm-3

ii) Calculate the total number of moles of HCl in the tank. Give your answer to 3 sig.fig.

400m3 = 40dm3

mol = 40 x 0.250 = 10 mol of HCl

b) Calculate the mass in tonnes of slaked lime required to neutralise the acid. Slaked lime reacts with hydrochloric acid according to the following equation: Ca(OH)2 + 2HCl --> CaCl2 + 2H2O

1Ca(OH)2 : 2HCL

mol = 10 x 2 = 20 mol Ca(OH)2

20 = mass / 74.1 = 1482g = 0.00148 tonnes of Ca(OH)2 needed

c) The slaked lime was manufactured by roasting limestone and then adding water:
CaCO3 --> CaO + CO2
CaO + 2H2O --> Ca(OH)2
Calculate the mass of limestone required to produce 1kg of slaked lime.

I have no idea how to do this one
2. You did all the hard ones, the last one is probably the easiest!

First work out how many moles of Ca(OH)2 you need to make.

Use Mass = Mr x Mol, where mass = 1kg

Spoiler:
Show
1000 = (40.1 + 34) x mol

mol = 1000/(40.1+34)

mol = 13.4953...

Use this answer to work out the moles of Limestone needed

Spoiler:
Show
Everything relevant uses a 1:1 mole ratio, so there is 13.4953 moles of Limestone required.

Now using moles of Limestone to work out mass

Spoiler:
Show
mass = mr x mol

mass = 13.4953... x 100.1
= 1350g

= 1.35kg

Hope this helps
3. (Original post by Dylann)
You did all the hard ones, the last one is probably the easiest!

First work out how many moles of Ca(OH)2 you need to make.

Use Mass = Mr x Mol, where mass = 1kg

Spoiler:
Show
1000 = (40.1 + 34) x mol

mol = 1000/(40.1+34)

mol = 13.4953...

Use this answer to work out the moles of Limestone needed

Spoiler:
Show
Everything relevant uses a 1:1 mole ratio, so there is 13.4953 moles of Limestone required.

Now using moles of Limestone to work out mass

Spoiler:
Show
mass = mr x mol

mass = 13.4953... x 100.1
= 1350g

= 1.35kg

Hope this helps

Thank you so much! haha I was totally overcomplicating it

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