Chemistry: Post your doubts here!

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Sameee......i'm not even confident about my answers
I did 12, and it was horrible as well.
The questions needed alot of time, and most questions were organic.
If we had more time I'd say it was good.
I had no time to check I just answered.
 
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Q: Equal masses of barium carbonate and magnesium carbonate powders are mixed together. The mixture is then heated using a Bunsen burner flame until there is no further change. A gas is given off. Which statements are correct?
1) The residue left after heating reacts with aqueous hydrochloric acid to produce carbon dioxide.
2) The percentage decrease in mass after heating is 26% (to 2 significant figures).
3) The gas given off during heating relights a glowing splint.

1. The fact that some carbon dioxde is produced when the residue is treated with acid means that there is a carbonate present.

2. If 50% of the mixture is magnesium carbonate (relative mass = 84) and it decomposes to MgO (relative mass 40), then the 50% loses 44/84 x 100 % of its mass = 52.4%
so the mass loss of the 100% mixture = 26.2%( dont get this part)


How from 52.4 become 26.2?
 
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Q: Equal masses of barium carbonate and magnesium carbonate powders are mixed together. The mixture is then heated using a Bunsen burner flame until there is no further change. A gas is given off. Which statements are correct?
1) The residue left after heating reacts with aqueous hydrochloric acid to produce carbon dioxide.
2) The percentage decrease in mass after heating is 26% (to 2 significant figures).
3) The gas given off during heating relights a glowing splint.

1. The fact that some carbon dioxde is produced when the residue is treated with acid means that there is a carbonate present.

2. If 50% of the mixture is magnesium carbonate (relative mass = 84) and it decomposes to MgO (relative mass 40), then the 50% loses 44/84 x 100 % of its mass = 52.4%
so the mass loss of the 100% mixture = 26.2%( dont get this part)


How from 52.4 become 26.2?
In this case, the answer must be D.
Since they mentioned bunsen burner, we know that MgCO3 is the one that went through decomposition, not barium compound since as you go down the group heir thermal stability increases and will require more energy that a burner cannot provide. The residue left is BaCO3 which will obv react with HCl to give out CO2.
For option 2, you are right, % decrease, in this case, is 52.19.. and not 26% therefore option 2 is wrong. You calculate % decrease by (original-final/original)*100.
3 is obviously wrong. I am assuming you also answered D! :)
 
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Ethanal, CH3CHO, can be reduced using an aqueous methanolic solution of NaBH4, as the reducing agent.
This is a nucleophilic addition reaction.
What could be the first step of this mechanism?
A) attack of an H+ ion at the carbon atom of the carbonyl group
B) attack of an H+ ion at the oxygen atom of the carbonyl group
C) attack of an H- ion at the carbon atom of the carbonyl group
D) attack of an H- ion at the oxygen atom of the carbonyl group

Can someone explain why the ans is C?
 
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Hey guys, A level electrolysis question up ahead-
1568479070641.png

the question is on the right and the mark scheme on the left. According to my calculations, 1519.685 C of charge was used and therefore time = 1519.685/0.5 which is also what they said. My answer is, however, t=3039 seconds and NOT 5034 seconds. So is the mark scheme wrong in this case? or is there something that I am overlooking?
 
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