VCE Chemistry Question Thread

[RazzMeTazz]

What is the primary difference between gavimetric and volumetric analysis? Thanks

I would say, gravimetric analysis is a form of quantitative chemical analysis based on mass measurement, whilst volumetric analysis is a form of quantitative chemical analysis based on the volumes of solutions, and their concentration.

Hope that was what you were looking for!

[RazzMeTazz]

Why would gravimetric analysis often be used in conjunction with other quantitative techniques?

Thanks

[wunderkind52]

Out of gravimetric and volumetric analysis, which technique would provide more accurate results ?

Thanks

Gravimetric analysis is more accurate, however it takes longer.
Many of the errors associated with volumetric analysis - overshooting the endpoint, incorrect rinsing, loss of sample, parallax error) are not associated with gravimetric analysis. This might be a little far-fetched, but volumes of solutions can be affected by temperature. Hope that answers your question.

[RazzMeTazz]

For this question:

The moisture content of an area of soil is one of its most important properties. A simple method for determining this is to take a soil sample and heat it in an oven at 105°C until constant mass is achieved. In a particular experiment, the following results were obtained.

Initial mass 124.829 g
Mass (after 60 min) 115.813 g
Mass (after 70 min) 109.614 g
Mass (after 80 min) 107.214 g
Mass (after 90 min) 107.212 g
Mass (after 100 min) 107.214 g

(a) Calculate the percentage of moisture in the soil sample.

To find the m(H2O) the answer seems to have done: 124.829 = 107.212

How would you know to use the value of 107.212 , rather than 107.214 (Since this is the value that occurs at the final heating, after 100min)

In these types of questions, would you always just choose the lowest value for the mass of the dehydrated substance (in this case 107.212)

Thanks!

[Kel9901]

For this question:

The moisture content of an area of soil is one of its most important properties. A simple method for determining this is to take a soil sample and heat it in an oven at 105°C until constant mass is achieved. In a particular experiment, the following results were obtained.

Initial mass 124.829 g
Mass (after 60 min) 115.813 g
Mass (after 70 min) 109.614 g
Mass (after 80 min) 107.214 g
Mass (after 90 min) 107.212 g
Mass (after 100 min) 107.214 g

(a) Calculate the percentage of moisture in the soil sample.

To find the m(H2O) the answer seems to have done: 124.829 = 107.212

How would you know to use the value of 107.212 , rather than 107.214 (Since this is the value that occurs at the final heating, after 100min)

In these types of questions, would you always just choose the lowest value for the mass of the dehydrated substance (in this case 107.212)

Thanks!

yes you always choose the lowest value as it is the one with the least amount of water (hopefully none)- water could've re-entered the sample somehow and increased the mass for the 100 min one

[RazzMeTazz]

Thanks Kel9901! That makes sense

[RazzMeTazz]

Gravimetric analysis is more accurate, however it takes longer.
Many of the errors associated with volumetric analysis - overshooting the endpoint, incorrect rinsing, loss of sample, parallax error) are not associated with gravimetric analysis. This might be a little far-fetched, but volumes of solutions can be affected by temperature. Hope that answers your question.

Oh okay! Yeah that answers my question, thanks

[StressedAlready]

Will ask the dumb question here:

The total number of atoms in 3.50g of ethane (C2H4) is:
A)   0.125
B)   0.750
C)   7.53 x 10²²
D)   4.52 x 10²³

WHY DO I KEEP GETTING 1.26 x 10^25?!
Aren't you supposed to use the formula:
N = n x N_A

[RazzMeTazz]

Did you mean ethene?

 n(C2H4) = m / M
                = 3.50 / 28
                = 0.125
   
n(atoms) = n(C2H4) * 6
                 = 0.125 * 6
                 = 0.75
and then yes you can use your formula of N = n * NA

So, that would be 0.75* 6.02 * 10^23 = 4.52 * 10^23

Hope that helps
 

[RazzMeTazz]

I wasnt quite sure or questions b.)i and b.) ii

Any help would be appreciated

[RazzMeTazz]

I wasnt quite sure how to do question
 b.) iii

The answer is : Fe2O3(s) + 6H+(aq) --> 2Fe3+(aq) + 3H2O(l)
 
But I didnt really know how you would know what products are formed.
Thanks

[lzxnl]

You need to think of the oxide ion in iron(III) oxide as a base, which it is. It then reacts with any protons to form water.

This works in general, although other metal oxides behave weirdly (you don't need to worry about those).

[RazzMeTazz]

Thanks lzxnl!

A secondary standard is defined as: a solution whose concentration has been derived from a primary standard.

So, would it be correct refer to a solution whose concentration has been calculated (through titration with a standard soution) as a secondary standard? Could you also refer to a diluted solution of  a standard solution as a secondary standard?

Thanks!

[wunderkind52]

Thanks lzxnl!

A secondary standard is defined as: a solution whose concentration has been derived from a primary standard.

So, would it be correct refer to a solution whose concentration has been calculated (through titration with a standard soution) as a secondary standard? Could you also refer to a diluted solution of  a standard solution as a secondary standard?

Thanks!

A secondary standard has a concentration accurately determined by titrating it against a known solution. So yes, NaOH is a secondary standard (bad primary standard).

I don't THINK diluting a standard solution will make it a secondary solution...because you would originally add water to the primary standard to make it a primary standard solution in the first place. Not sure about that one though.

[Eiffel]

I have a question.

Question is, a student monitored the changes in ph that occured when 20ml of base x is titrated with 0.1m hcl. curve a obtained (ph begins around 13 and is sharp enpoint with v(hcl) being 19.14ml).

the student then titrated 20ml of a second base, y with the same acid solution and obtained curve b (sharp end point ph 11.2 and titre 24.27)

Can someone explain which one is more concentrated and why, it says base y because larger titre, but why is this the case (i know we need more hcl to neutralise but i thought this whole thing deals with mols?)

and then it says base x is the stronger one (i assume because larger ph) but why is the concentration and ph in this case not intrinsically linked.

thanks