Qualification > Sciences
IGCSE CHEMISTRY DOUBTS
J.Darren:
By the way the more reactive metal is a better reudcing agent becuase it has a greater tendency to form positive ions, whereas the relatively weaker metal is a better oxidising agent as it is the best at accepting electrons.
Allyza1n:
hey...
Question 5 b from chemistry may june 2009 variant 1
Vin:
--- Quote from: Ivo on June 04, 2010, 12:40:43 pm ---
7) b) i) Moles of Mg = 3/24 = 0.125
Moles of CH3COOH = 12/60 = 0.200
Using ratios given in the equation, to compare, divide 0.2/2 = 0.100 moles of CH3COOH.
Therefore, magnesium is in excess.
ii) The one not in excess: ie. 0.1
--- End quote ---
Hey Ivo, can you tell me in detail how did you get the part in bold^^. I'm totally confused with this chapter. Also do you have any rules/notes based on this chapter. Your help is much appreciated! :)
Ivo:
--- Quote from: ~VIN1094~ on June 05, 2010, 10:43:20 pm ---Hey Ivo, can you tell me in detail how did you get the part in bold^^. I'm totally confused with this chapter. Also do you have any rules/notes based on this chapter. Your help is much appreciated! :)
--- End quote ---
OK, we know from the equation given, the mole ratio: 1 mole of magnesium reacted with 2 moles of ethanoic acid.
The amounts of substance actually used was: moles of magnesium = 0.125, moles of ethanoic acid = 0.2. But from above, you see the ratio 1:2. So here, we either divide 0.2/2=0.1 or multiply 0.125 by 2 = 0.25.
Either way, you'll find that the magnesium is in excess and the ethanoic acid the limiting reagent. Has this helped?
For the second part, we are also using the mole ratio from the equation where 2 moles of ethanoic acid gives 1 mole of hydrogen. We can't use the calculations for magnesium as this was in excess, so we use the limiting reagent (ie. ethanoic acid), so 0.2/2=0.1.
And no sorry, don't have any notes. I guess, as long as you can handle this question, I think you'll be fine. I don't think it gets any harder than this.
Ivo:
Also, can someone please explain the colour changes and what happens for the potassium manganate and potassium iodide, I have no idea what they mean. Thanks in advance.
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