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    (Original post by panda14)
    A or B? (Basically Salters or the other one)

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    A
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    Right can someone explain this question to me.

    When you do the normal sum of products-sum of reactants it works.

    But when I try with a Hess cycle I don't get the right answer. Anyone care to explain this to me?
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    i have my AS Titration coming up soon. dont know how to prepare for it.
    i had my AS thermal decomposition one today, didnt go so well. any tips on how to ace the titration isa
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    (Original post by Ray Robinson)
    i have my AS Titration coming up soon. dont know how to prepare for it.
    i had my AS thermal decomposition one today, didnt go so well. any tips on how to ace the titration isa
    make sure you are so comfortable with all the stoichiometry and every kind of molar calculation. Make sure you're familiar with how to record your results, paying attention to consistent decimal places that are close in range and always the same number of decimal places, I advise 2 d.p . Also be familiar with the pH indicators and their colour changes. Make sure you're also comfortable with writing equations, and all types of acid base reactions. something to note is that even if your results are really inconsistent don't record them like that, so many people did that for some reason, I think there's a mark specifically for obtaining titres within 0.01cm3 range, but this is all based on OCR A by the way. Good luck!


    AS student, I got 100% in my titration practical
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    I really really really need help with an upcoming evaluative assessment retake, it make the difference of a whole grade for me. I'm AS OCR A, the assessment is on enthalpy change of combustion. I need to know what I should expecting... I got 100% in the other two im sooo nervous. Thank you so much
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    (Original post by AyshaU)
    Like you said to me when we were talking about organic, how the hell can you love amount of substance, its an okay topic and I'm sure I've revised pretty much every question that is out there as I've started doing the same papers again. I know it comes in every paper and that scares me. I've just got bonding to cover and I have finished my CHEM1 revision. then onto CHEM2 (yay!!)

    If iI' not being nosy can I just ask what other A-levels did you do and how much time did you spend on each of these per week.
    If you don't want to answer I honestly would understand
    I guess the reason why I love Amount of Substances is because I love Maths and for me Amount of Substances is the best way I can exercise my Maths skills in a different subject.

    No you're not being nosy at all

    I student Maths, Physics and of course Chemistry.

    I dropped English Literature at AS because I knew I would've neglected putting effort into it until the end of the year, even though I loved the subject, I didn't want to put that kind of stress on myself as I knew deep down I will always prioritise Maths and Sciences.

    Uh outside school hours I'd spend at least 4.5 hours a day in general for school work but most of the time I use that time to do Maths homework because my teacher gives my class a crap load to do everyday

    I cannot give you an approx figure as to how many hours I spend for each subject because how I use my time depends on how much work I get given from my teachers and whether I'll have enough time after to do my own independent revision.

    But I think its safe to say most of my weekend is revision and consolidating time.

    I hope that helps.

    Now I hope I'm not being nosy by asking what other A - Levels you do.
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    (Original post by Super199)
    Right can someone explain this question to me.

    When you do the normal sum of products-sum of reactants it works.

    But when I try with a Hess cycle I don't get the right answer. Anyone care to explain this to me?


    You're mistake in the picture is that you flipped both the arrows up, I believe you did this because they told you to find ΔHf so you must have assumed that after drawing the Combustion Hess' Cycle you'd then have to flip both sides up. But this implies that you're thinking all the values in the data are ΔHf values (if you were to take them as positive values).

    Here's a tip when dealing with examination questions on Hess' cycles: if for example they ask you to find ΔHc of a certain reaction but the data they give you is ΔHf values then you must draw a Formation Hess' Cycle. But do not flip both arrows down just because they're asking for ΔHc, just flip the left arrow down so that the cycle can go in the right direction.



    The picture above should help.

    Your aim is to get ΔHc = -ΔHf2 + ΔHf1 OR ΔHc = ΔHf1 - ΔHf2

    I hope that helps, let me know if you need any more help understanding this.
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    (Original post by Disney0702)

    Ah brilliant! Thank you for this. Just another thing you know the bit at the bottom where you CO2 + H20. Is that the products for combustion so when they give you a hess cycle and you have combustion it would be those. Do they need to be balanced? Also what would it be if it was formation instead (the things at the bottom). If that makes any sense??

    Thanks again
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    what is the best way to revise?
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    (Original post by Super199)
    Ah brilliant! Thank you for this. Just another thing you know the bit at the bottom where you CO2 + H20. Is that the products for combustion so when they give you a hess cycle and you have combustion it would be those. Do they need to be balanced? Also what would it be if it was formation instead (the things at the bottom). If that makes any sense??

    Thanks again
    Ah you're welcome

    Yes, the CO2 + H2O at the bottom are the combustion products formed when you're drawing a Combustion Hess' Cycle. And no, I do not think they need to be balanced, that's what my teacher always told me last year, but just to be on the safe side I'd say just make it balanced because it can only benefit you if you do.

    As for the Formation Hess' Cycle you write the constituent elements that the reactants and products are made from and again I think it's best if you balance that out as well.



    I hope the picture above helps clarify your concern.

    Please let me know if you need anything else
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    (Original post by Aliciaalicia)
    what is the best way to revise?
    Hi there!

    I'm an A2 AQA Chemistry student and I personally found the best way for me to revise during AS was to tackle all the past paper questions.

    This not only helped strengthen my understanding on topics I found difficult but it also helped me understand the language of the exam. I have found sometimes that the way AQA phrase their questions just puts me on the spot and makes me question as to whether I was taught that topic.

    Well as usual, yes I was taught that topic, it's just some of their questions prompts you to think outside the box from time to time and I had to learn how to do that when given an unexpected question.

    I also used my CGP AQA AS Chemistry Student book to help me and it was amazing, I totally recommend all students to get it.

    CGP Student book

    Sample Pages
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    (Original post by Joshthemathmo)
    When silicon tetrachloride is added to water, the following reactionoccurs:
    SiCl4(l)+ 2H2O(l)SiO2(s)+ 4HCl(aq)
    1.2g of impure silicon tetrachloride was dissolved in excess water, andthe resulting solution was made up to 250 cm3.A 25 cm3portion of the solution was then titrated against 0.10 moldm-3sodium hydroxide, and 18.7 cm3of the alkali were required. What was the percentage purity of thesilicon tetrachloride?

    I've been stuck on this for a while now. Any help would be appreciated




    Oh! I realised that in the second image I didn't put the total value of moles of the silicon tetrachloride when solving for mass, nor did I round it up properly to 4.68 x 10-3 .

    Had I kept the correct figures the answer would've become 66.3%.
    So I guess you can take this as a lesson to never rush through your working out even if it is a long method :^_^:

    Well I hope that helps.

    Please excuse the poor picture quality, my camera isn't that good , I hope you can still read what I have done.

    Let me know if you still need any help.
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    How would I do 3. (a)(I)?
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    So your given 1.8dm^3 and 1 mole of a gas occupies 24dm^3

    So you need to find how many moles occupy 1.8dm^3.

    Thanks. James.
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    (Original post by jamesgates1)
    So your given 1.8dm^3 and 1 mole of a gas occupies 24dm^3

    So you need to find how many moles occupy 1.8dm^3.

    Thanks. James.


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    24 /1.80=13.3 moles?
    If that's the answer,sounds very big for moles.
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    You expect to see less than 1 mol as 1.8 is less than 24. Therefore it is 1.8 / 24
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    Posted from TSR Mobile

    Ty.How do you convert from cm to dm3?
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    (Original post by Kadak)
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    Ty.How do you convert from cm to dm3?
    Do you mean convert cm3 to dm3?

    If that's what you meant then, 1 cm3 = 1x10-3 dm3
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    Posted from TSR Mobile

    Question (II)
    Is this the 3 correct isomers?
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    Can someone help me with this question.

    A 25cm3 sample of vinegar was diluted in a 250cm3 volumetric flask. This was then put in a burette and 23.10cm3 of the diluted vinegar neutralised 25cm3 of 0.1M NaOH. What is the concentration of the vinegar in gdm-3.

    CH3CO2H + NaOH --> CH3CO2-Na + H2O.

    Right what I first did is workout the moles of sodium hydroxide which was 0.00250 moles.
    Moles of that is equal to the CH3CO2H, so I worked out the concentration of that which was 0.108dm-3. Then I hit a brick wall. Anyone care to explain what I have to do?
 
 
 
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