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# AQA - Unit 5 - Energetics, Redox and Inorganic Chemistry watch

1. (Original post by Spongy)
hi, can someone help with this question:

its the february 2010 paper, question 8(c):

A chemical company has a waste tank of volume 25 000dm3. The tank is full of
phosphoric acid (H3PO4) solution formed by adding some unwanted phosphorus(V)
oxide to water in the tank.
A 25.0cm3 sample of this solution required 21.2cm3 of 0.500 mol dm–3 sodium
hydroxide solution for complete reaction.
Calculate the mass, in kg, of phosphorus(V) oxide that must have been added to the
water in the waste tank.

this is what the mark scheme says:

Moles NaOH = 0.0212 × 0.5 = 0.0106
Moles of H3PO4 = 1/3 moles of NaOH (= 0.00353)
Moles of P in 25000 l = 0.00353 × 106 = 3.53× 103
Moles of P4O10 = 3.53 × 103/4
Mass of P4O10 = 3.53 × 103/4 × 284 = 0.251 × 106 g
= 251 kg

I dont get how they worked it out . I undestand the first line of the mark scheme and that is it.
write an equation of what happens when NaOH reacts with H3PO4
2. (Original post by ?!master?!mini?!)
write an equation of what happens when NaOH reacts with H3PO4
what is that equation ???? -> i dont know it
3. (Original post by Spongy)
what is that equation ???? -> i dont know it
well without it you're pretty stuffed.

Its
H3PO4 + 3NaOH --> Na3PO4 + 3H2O

we know the number of moles of naoh.

how do we find the number of moles of h3po4
4. (Original post by ?!master?!mini?!)
well without it you're pretty stuffed.

Its
H3PO4 + 3NaOH --> Na3PO4 + 3H2O

we know the number of moles of naoh.

how do we find the number of moles of h3po4
i can work the rest out now !!!

i couldn't find the equation in my notes anywhere! or in the textbook!
5. (Original post by Spongy)
i can work the rest out now !!!

i couldn't find the equation in my notes anywhere! or in the textbook!
why do you divide the moles of P in 25000dm3 by 4?
6. (Original post by ?!master?!mini?!)
why do you divide the moles of P in 25000dm3 by 4?
this is the equation i do know

its

P4O10 + 6H20 -> 4H3PO4
7. (Original post by Spongy)
this is the equation i do know

its

P4O10 + 6H20 -> 4H3PO4
nice
8. (Original post by ?!master?!mini?!)
nice
How is your chem revision going, this unit seems to be a lot of memory work i think learning all the colours and equations etc.
9. (Original post by Spongy)
How is your chem revision going, this unit seems to be a lot of memory work i think learning all the colours and equations etc.
definitely lots of memory. tbh i prefer Unit 4 to Unit 5, even though all the people in my college prefer unit 5 to unit4.

It's coming along fine. Haven't really revised much. Chemistry is like second nature to me. I need not be complacent though. come to think of it, after finishing the biology questions that i'm doing right now, i'll do some chemistry questions.

How's yours?
10. i think this exam is the same as chem4 tbh but i do agree that learning the colours is quite hard
there must be an easier way to learn all the colours there must be

but the good thing about the colours is that the exam can only ask normal questions unlike biology so in that sense its ok
11. After having along break from CHEM 5 to revise Maths and Physics, time to get back to it. Yay *sigh*
12. literally cannot get into revision for this, I just don't know where to start
13. (Original post by ?!master?!mini?!)
definitely lots of memory. tbh i prefer Unit 4 to Unit 5, even though all the people in my college prefer unit 5 to unit4.

It's coming along fine. Haven't really revised much. Chemistry is like second nature to me. I need not be complacent though. come to think of it, after finishing the biology questions that i'm doing right now, i'll do some chemistry questions.

How's yours?
hey do you have any chemistry questions that i kud do
14. Haven't revised a bit for chem5, too stuck in chem4. Thank God for that alevel chemistry website with all the questions, makes revision much easier
15. It's a weird day when you realise that dissolving your sugar in your hot coffee is remarkably easier than dissolving sugar in cold water because the enthalpy of solution is endothermic and the equilibrium of the reaction shifts to the right at increased temperatures. And that since the enthalpy change is positive and the reaction is spontaneous, then entropy must also be positive for the free energy change to be negative, so there is an increase in disorder when you dissolve your sugar.

Cooking will never again be the same for me.
16. (Original post by jimmy303)
It's a weird day when you realise that dissolving your sugar in your hot coffee is remarkably easier than dissolving sugar in cold water because the enthalpy of solution is endothermic and the equilibrium of the reaction shifts to the right at increased temperatures. And that since the enthalpy change is positive and the reaction is spontaneous, then entropy must also be positive for the free energy change to be negative, so there is an increase in disorder when you dissolve your sugar.

Cooking will never again be the same for me.
LOL bless, I'm gonna use that in chem tmrw

:O I'm so sorry I meant to pos rep you! Stupid ipod sorry
17. (Original post by Miss Purple)
LOL bless, I'm gonna use that in chem tmrw

:O I'm so sorry I meant to pos rep you! Stupid ipod sorry
haha no worries
18. Does anyone else find that in the text book the colours seem to change from page to page?
19. (Original post by jimmy303)
It's a weird day when you realise that dissolving your sugar in your hot coffee is remarkably easier than dissolving sugar in cold water because the enthalpy of solution is endothermic and the equilibrium of the reaction shifts to the right at increased temperatures. And that since the enthalpy change is positive and the reaction is spontaneous, then entropy must also be positive for the free energy change to be negative, so there is an increase in disorder when you dissolve your sugar.

Cooking will never again be the same for me.
you're a first class G!
20. what raw mark do you need for an a in the isa?

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