VCE Chemistry Question Thread

[RazzMeTazz]

Help!

A 2 L sample of a gaseous hydrocarbon is burnt in excess oxygen. The only products of the reaction are 8 L of CO2(g) and 10 L of H2O(g), all at 100°C and 1 atm pressure.

The formula of the hydrocarbon is:

A. CH

B. C2H4

C. C4H10

D. C8H10

I would say the answer is C, since the volume ratio of these gases can essentially be taken as a mole ratio (since the pressure and temperature are constant)

So if you work this question out, taking into consideration that there is 8mol of CO2 and 10mol of H20 you can write an equation:
CxHy + O2 --> 8CO2 + 10H20

Then based on that, you can work out the number of mole of Carbon and Hydrogen respectively, then plug these values in to give you:

2C4H10 + 13O2 --> 8CO2 + 10H2O

Hope that helps

[vobinhood]

I would say the answer is C, since the volume ratio of these gases can essentially be taken as a mole ratio (since the pressure and temperature are constant)

So if you work this question out, taking into consideration that there is 8mol of CO2 and 10mol of H20 you can write an equation:
CxHy + O2 --> 8CO2 + 10H20

Then based on that, you can work out the number of mole of Carbon and Hydrogen respectively, then plug these values in to give you:

2C4H10 + 13O2 --> 8CO2 + 10H2O

Hope that helps

Hey thanks a lot!

I worked it out to be C as well. Your idea for the volume ratio being taken as the mole ratio because the pressure and temp are constant was so helpful!
However, your second idea about working out the moles of C and H to find the formula of the hydrocarbon confused me. Instead, I balanced the equation and found the common factors of 20 (number of H) and 8 (number of C) and that would be the coefficient of the hydrocarbon. The common factors were either 2 or 4. 4 doesn't work because the formula would result as C2H5 which isn't an option, which leaves me with 2, which gives C4H10, C!

[Eiffel]

a 0.9030 gram sample X(OH)2 was mixed with 20ml of 2M HCL and enough water added to make 100 ml of solution A (what is this?)

X(OH)2 + 2HCL -> 2H20 + XCl2

a 10ml aliquot of solution A was taken and titrated with 17.64ml of 0.05121M NaOH.

NaOH + HCL -> H20 + NaCl

find X, the metal (Mg)

HOW TO DO THIS???

Thanks

[vobinhood]

a 0.9030 gram sample X(OH)2 was mixed with 20ml of 2M HCL and enough water added to make 100 ml of solution A (what is this?)

X(OH)2 + 2HCL -> 2H20 + XCl2

a 10ml aliquot of solution A was taken and titrated with 17.64ml of 0.05121M NaOH.

NaOH + HCL -> H20 + NaCl

find X, the metal (Mg)

HOW TO DO THIS???

Thanks

I think you made a typo. Were you meant to write XCl2, in the second equation, instead of HCl?

[Eiffel]

nah its correct. sheet has HCl, i see where you are coming from though

[Eiffel]

also


calculate the concentration of hydroxide ions in a solution made from 25ml of 0.004M calcium hydroxide solutions and 10ml of 0.010M sodium hydroxide solution.

[warya]

Hey peeps

If I'm using gravimetric analysis to determine the percentage of CaCO3(s) in egg shell, and I'm dissolving it in HCl then I don't understand what the precipitate comes from (isn't that  the whole point?) if the salt produced is CaCl2(aq) which is soluble I'm sure?

Or is it just that the egg shell is dissolved in HCl and then later precipitated with something else? If so, then why is it dissolved in HCl and not water as usual?

I'm so confused lol

[lzxnl]

Hey peeps

If I'm using gravimetric analysis to determine the percentage of CaCO3(s) in egg shell, and I'm dissolving it in HCl then I don't understand what the precipitate comes from (isn't that  the whole point?) if the salt produced is CaCl2(aq) which is soluble I'm sure?

Or is it just that the egg shell is dissolved in HCl and then later precipitated with something else? If so, then why is it dissolved in HCl and not water as usual?

I'm so confused lol

You wouldn't use HCl to precipitate it. You'd probably use something like H2C2O4 (oxalic acid) which both acts as an acid and forms an insoluble oxalate salt with the calcium.

[warya]

So why use HCl to dissolve? Like does it have a specific purpose or could it have been water? They gave us the mole of HCl so I'm assuming it'll be used in some way??

[warya]

Wait is it that the CaCl2 is precipitated against H2C2O4 to form the precipitate CaC2O4 which is insoluble?
Therefore the purpose of the HCl is to form the CaCl2?

But couldn't the Calcium Carbonate have been precipitated with the Oxalic acid by itself why the extra step

[knightrider]

How would you do this question?


Name the period and group of the following element

The element with a +3 charged ion that has an electronic configuration of

[lzxnl]

Wait is it that the CaCl2 is precipitated against H2C2O4 to form the precipitate CaC2O4 which is insoluble?
Therefore the purpose of the HCl is to form the CaCl2?

But couldn't the Calcium Carbonate have been precipitated with the Oxalic acid by itself why the extra step

The purpose of the HCl is to dissolve the calcium. The experiment, however, is flawed as oxalic acid would be so much easier to use. HCl isn't the right precipitating reagent here.

How would you do this question?


Name the period and group of the following element

The element with a +3 charged ion that has an electronic configuration of

+3 charged ion? Add 3 more electrons to find the number of protons in this element.
Currently we have 10 electrons. Add three to find the neutral atom electron count, we get 13 electrons. Periodic table should tell you that this is aluminium, which is period 3 group 13.

[RazzMeTazz]

In the process of gravimetric analysis, what would be the purpose of cooling the precipitate in a dessicator after it has been heated to constant mass? (This step is outlined in my textbook as a step that is involved in gravimetric analysis, but I am unsure why).

Thanks

[knightrider]

+3 charged ion? Add 3 more electrons to find the number of protons in this element.
Currently we have 10 electrons. Add three to find the neutral atom electron count, we get 13 electrons. Periodic table should tell you that this is aluminium, which is period 3 group 13.

Thanks Lzxnl 

[sunshine98]

In the process of gravimetric analysis, what would be the purpose of cooling the precipitate in a dessicator after it has been heated to constant mass? (This step is outlined in my textbook as a step that is involved in gravimetric analysis, but I am unsure why).

Thanks

A desiccator is a sealed container , containing a desiccant ( a hygroscopic substance which attracts moisture and water from surrounding environment ) , essentially ,it  prevent the precipitate from absorbing moisture from the air, and also ensures that  all water molecules are removed.Despite the fact we say 'constant mass' , my teacher said that sometimes there can be still some water molecules present (we saw this in a question , where it said that it has reached constant mass , yet water was present in the formula  , as it was a hydrated salt)
Hope this helps