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Chemistry Forums for Students => High School Chemistry Forum => Chemistry Olympiad and other competitions => Topic started by: Evaldas on December 30, 2010, 01:30:52 PM

Title: Olympiad question #2 (Salts)
Post by: Evaldas on December 30, 2010, 01:30:52 PM: 10 g iron plate was submerged into 200 g 8% copper(II) sulfate solution. After some time the plate was taken out and it got heavier by 0,8 g. Find out:
a) did all the copper(II) sulfate react?
Reaction:
CuSO₄(aq) + Fe(s) :rarrow: FeSO₄(aq) + Cu(s)
m(CuSO₄) = 16 g.
What's next?
n(Fe)=0.17857 mol; n(CuSO₄)=0.1 mol?
All the copper sulfate reacted because there was excess of iron?
Title: Re: Olympiad question #2 (Salts)
Post by: Borek on December 30, 2010, 02:58:31 PM: How does mass of the plate change when 1 mol of iron reacts?
Title: Re: Olympiad question #2 (Salts)
Post by: Evaldas on December 30, 2010, 03:08:19 PM: Quote from: Borek on December 30, 2010, 02:58:31 PM
How does mass of the plate change when 1 mol of iron reacts?
Gets replaced with 1 mol of copper?
Title: Re: Olympiad question #2 (Salts)
Post by: Borek on December 30, 2010, 07:00:21 PM: Mass of the plate gets replaced with 1 mole of copper?

Think again. This time start by reading the question I asked.
Title: Re: Olympiad question #2 (Salts)
Post by: Evaldas on December 31, 2010, 04:26:17 AM: Gets heavier because copper is heavier than iron?
Title: Re: Olympiad question #2 (Salts)
Post by: Borek on December 31, 2010, 06:08:40 AM: By how much?
Title: Re: Olympiad question #2 (Salts)
Post by: Evaldas on December 31, 2010, 06:25:40 AM: If we're speaking about 1 mol of each then the difference between M(Fe) and M(Cu), 64g/mol-56g/mol=8g/mol.
Right?
Title: Re: Olympiad question #2 (Salts)
Post by: Schrödinger on December 31, 2010, 06:46:14 AM: Yes. Correct :)
Title: Re: Olympiad question #2 (Salts)
Post by: Evaldas on December 31, 2010, 07:36:53 AM: This is what solution they suggest (saying that it is one of the many possible):
CuSO₄ + Fe :rarrow: FeSO₄ + Cu
Reacted: x mol x mol
Formed: x mol x mol
Reacted iron mass: 56x, formed copper mass 64x
:delta: m = -56x + 64x = +0,80
from here x = 0,1 mol. That means 0,1 mol of CuSO₄ reacted (which is 16 g) and 0,1 mol of FeSO₄ formed (which is 15,2 g)
There were 16 g of CuSO₄ in the solution so it means all the CuSO₄ reacted.
What do you think about this solution?
Do you think this exercise is too easy/difficult for students who are in the last two years of high school chemistry?
Title: Re: Olympiad question #2 (Salts)
Post by: Borek on December 31, 2010, 11:41:23 AM: You have checked that 8g mass difference means 1 mole reacted, next step is to answer the question - if 8g change in mass means 1 mole reacted, how many moles reacted if mass changed by 0.8g?

That's the same approach they used, they too combined mass difference with number of moles.
Title: Re: Olympiad question #2 (Salts)
Post by: Evaldas on December 31, 2010, 12:12:09 PM: Quote from: Borek on December 31, 2010, 11:41:23 AM
You have checked that 8g mass difference means 1 mole reacted, next step is to answer the question - if 8g change in mass means 1 mole reacted, how many moles reacted if mass changed by 0.8g?

That's the same approach they used, they too combined mass difference with number of moles.
0,1 mol