I respect your standpoint, and would actually prefer the
guidance in answering questions more than offering worked
solutions. For example, if you could choose a question of each type
[like there was a limiting reactant and water of crystallisation
question in the previous topic posted] and give me a guided
solution for that one specific question then that would be very
helpful and much appreciated.
In the previous topic, the solutions were laid out far more
effectively than mine were and so I was obviously making it more
difficult for myself by not having a logical and considered
approach.
I'm slightly - but not considerably - older than the age
expected. [Thank you, by the way. I'll take that as a compliment.]
I'm from the UK, studying in the UK, and will be sitting my exams
in the UK. I can't afford to go to College either full-time or
part-time, and so I'm working full-time and having to complete my
studies around this. AQA is the certifying exam board for my
Chemistry course, which is an A-level and so considered A-level
standard. [I'm not finding it difficult in terms of getting my
answers correct, but I've been out of learning as I've not come
straight from GCSE and so I'm not well-drilled at organising my
methods and thinking logically about problems all of the time]. I
intend on studying Biochemistry at University and I'm taking other
A-levels via distance-learning that ensure I've got the
prerequisites for entry [I could have taken other routes into
University but they were more intensive in the short-term and I
couldn't balance it with my worklife].
I'll just post now and again if you don't mind, and if someone
wants to give some input then they can know that I'll appreciate
it, but if someone doesn't wish to contribute for whatever reason
then I completely understand.
Thanks for your time in reading this.
Hi Tullia, thanks for your self-intro, and yes, feel free to post
your questions here every now and then.
Although here's a little tip : perhaps one or two questions
at a time, might be less of a turn-off for forumners who might
otherwise be willing to reply your (one or two) questions.
As I said earlier, I'll only give brief comments to guide your
thinking (if anyone else reading the thead feels like contributing,
don't refrain on account of my
comments-only-instead-of-worked-solutions, go ahead and post the
full worked solutions if you feel like it), and if my comments
include any phrases you're not familiar with (eg. "molarity" or
"aliquot" or "Kekule structure" or "formal charge" or
"oxidation state", etc), you're advised to google and (in
particular) Wikipedia them out for your own self-learning.
And it helps for forumners to know that you've skipped O
levels and are studying A levels from scratch (effectively,
self-studying O and A levels simultaneously). Knowing that you're
ultimately taking the AQA 'A' level exam (in 12 months, I presume?)
allows me to more freely use 'A' level terminology (although I
personally often go somewhat beyond the 'A' level
syllabuses when teaching my own 'A' level students, so be
mentally prepared to do a substantial amount of googling and
wikipeding when reading my comments) in my comments for you.
Firstly, remember that in chemistry calcuations, algebra is your
best friend. The first couple of questions need not involve
algebra, but as the questions become more involved, during your 'A'
level course, you'll find increasing need for the use of
algebra.
Q4. Calculate moles of H2SO4. Since it's a diprotic acid, hence
calculate moles of protons H+ involved, which is also the same
as the moles of hydroxide ions OH- involved, which in turn is
the same as the moles of NaOH involved. Now that you've the moles
of NaOH, and the Qn gives you volume of
NaOH, hence determine the molarity of NaOH, commonly
written as [NaOH], using the formula : Molarity = Moles / Volume
(in dm3).
Q5. Calculate moles of HNO3 present. Since both acid and base
are monoprotic, hence determine moles of KOH required.
Since molarity of KOH is given, hence determine volume
required, using the formula : Molarity = Moles / Volume (in
dm3).
Q6. Calculate moles of HCl neutralized, which would be the same
as moles of OH- present in the 25cm3 NaOH(aq) aliquot, hence
calculate the moles of OH- present in original 250cm3 of
NaOH(aq) solution (from which the aliquot was obtained), thereafter
multiply by the molar mass of NaOH to determine the sample mass of
NaOH required by the qn.
Q7. Let x be the volume (in dm3) of HCl required. Given the
molarity of HCl, calculate moles of HCl involved, in algebraic
terms. Given sample mass of CaCO3, calculate the moles of CaCO3
(simply divide the sample mass by molar mass). Bearing in mind that
the CO3 2- ion is diprotic, hence calculate the moles of HCl
required for acid-base neutralization. Equate this value to the
moles of HCl in algebraic terms found earlier, and solve for x.
Q8. Let x be the molar mass of the monoprotic acid. Given sample
mass, find moles of the acid (simply divide sample mass by molar
mass), in terms of algebraic variable x. Given 250cm3 volume,
determine molarity of acid in the 250cm3 solution. Since a 25cm3
aliquot was used for titration, calculate moles of H+ (still in
terms of x) involved in the titration. Given molarity and volume of
NaOH(aq) used in the titration, calculate moles of NaOH involved,
and equate to moles of H+ found earlier (in terms of x), and solve
for x.
Q9. Find molar mass of hydrated ethandioic acid, in terms of
algebraic variable n. Given sample mass, find moles of acid present
(in terms of n). Given volume of 250cm3, find molarity of acid
in 250cm3 solution, hence determine moles of diprotic acid
neutralized in titration (ie. in 25cm3 aliquot). Since ethandioic
acid is diprotic, hence determine moles of H+ neutralized in
titration (in terms of n). Equate this expression to the moles
of OH- neutralized in titration (use the formula
: moles = molarity x volume, for the NaOH), and solve for
n.
Q10. Let the molar mass of metal M, be x (grams). Accordingly,
determine moles of M2CO3 present, in terms of x (use the formula :
moles = sample mass / molar mass; note that sample mass is given,
and molar mass will be in terms of x). Given 1dm3 solution,
calculate molarity (still in terms of x). Given 25cm3 aliquot
involved in neutralization, determine moles of CO3 2- ions involved
in neutralization (use the formula : moles = volume x molarity).
Bearing in mind the carbonate(IV) ion CO3 2- is diprotic, hence
calculate moles of H+ neutralized (still in terms of x). Equate
this expression to the moles of HCl used (use the formula : moles =
molarity x volume), and solve for x.
Q11. Let the % by mass of CaCO3(s) present in the 1g sample be
x. Hence determine the sample mass of pure CaCO3(s) present, which
is simply (x/100)(1g). Divide this by the molar mass of CaCO3(s)
(refer to the periodic table) to obtain the moles of CaCO3(s)
involved in neutralization (in terms of x). Find
total moles of HCl present (moles = molarity x volume). Find
moles of excess HCl by subtracting twice the moles of CaCO3, from
the total moles of HCl present. (Why twice? because tghe
carbonate(IV) ion CO3 2- is diprotic!). Equate this expression
(of moles of excess HCl, in terms of x) with the moles of NaOH(aq)
used in titration (use the formula : moles = molarity x volume),
and solve for x.
Q12. Similar to Q11, but more involved and
convoluted.
Let the % purity, which is the same as % by mass of Ba(OH)2 in the
1.6524 g sample, be x. Hence find sample mass of pure Ba(OH)2
present, in terms of x grams. Hence find moles of Ba(OH)2 present
(formula : moles = sample mass / molar mass). Next find moles of
OH- present (still in terms of x), bearing in mind that Ba(OH)2 is
a diprotic base. Find total moles of HCl used (formula : moles
= 0.2M molarity x 0.1dm3 volume). Subtract away moles of OH-
present from total moles of HCl present, to obtain moles of
HCl in excess (still in terms of x).
Given molarity of HCl, find moles of HCl present in 28.5cm3.
This would give you the moles of H+ involved in the other
titration, which is the same as the moles of OH- involved in this
other titration. Given that in this other titration, volume of OH-
used was 25cm3, hence determine molarity of NaOH(aq). Hence
determine moles of OH- present in 10.9cm3. Equate this value, to
the moles of HCl in excess (in terms of x) found earlier, and
solve for x.