VCE Chemistry Question Thread

[knightrider]

How would you determine the density,hardness and magnetism of metals?

[keltingmeith]

How would you determine the density,hardness and magnetism of metals?

... This is so geology, lel. This definitely from chem?

Density=mass/volume, so weigh it and check against dimensions.

Hardness: try scratching it, compare using Mohs hardness scale.

Magnetism: I'm assuming you mean if it is magnetic? Put it next to a non-magnetic metal, see what happens.

[cosine]

Hi cosine, the mole ratio in reference to compounds is the amounts of moles of any two compounds involved in a chemical reaction, for example, 2H2(g) + O2(g) → 2 H2O(g) where every 1 mole of O2 used, 2 moles of H2Ore formed. Therefore the mole ratio between O2 and H2O is 1:2.

Does this also mean that for every 1 mole of Oxygen to reaction, you need 2 moles of Hydrogen to accompany it?

[grannysmith]

In your previous post you stated that the calculated HCl concentration would be lower. That's why I asked the question.

The problem with this is that the proposed amount of Na2CO3 doesn't change in our working. Thus our calculations will not reflect the actual decrease in the amount of Na2CO3 (due to inadvertent dilution). Consequently, we will obtain a lower titre volume (less HCl is needed to neutralise the smaller amount of Na2CO3) and thus a greater perceived concentration of the HCl solution.

Okay so Na3CO3 aliquot is diluted. Lower [Na2CO3] means lower titre volume. But apparent n(Na2CO3) remains the same = n(HCl). So higher [HCl]. You're right, sorry. I was confusing myself haha

[lzxnl]

How would you determine the density,hardness and magnetism of metals?

You can measure its magnetic susceptibility using a variety of instruments (google it)

Does this also mean that for every 1 mole of Oxygen to reaction, you need 2 moles of Hydrogen to accompany it?

Yes. For every oxygen molecule, you need two hydrogen molecules present for the reaction. Both will disappear and produce two water molecules.

[Kel9901]

In your previous post you stated that the calculated HCl concentration would be lower. That's why I asked the question.

The problem with this is that the proposed amount of Na2CO3 doesn't change in our working. Thus our calculations will not reflect the actual decrease in the amount of Na2CO3 (due to inadvertent dilution). Consequently, we will obtain a lower titre volume (less HCl is needed to neutralise the smaller amount of Na2CO3) and thus a greater perceived concentration of the HCl solution.

let me think about it again, i may have confused myself lol

[FarAwaySS2]

Can someone please help me with these questions? The answers are A,A.

Thanks!

[grannysmith]

Can someone please help me with these questions? The answers are A,A.

Thanks!

16. HCl is limiting reagent, hence all of it reacts.
n(AlCl3) = 1/3 n(HCl) = 1 mol.
n(H2) = 1/2 n(HCl) = 1.5 mol.
Note that Al is the excess reagent and we only react 1 mole of it. There's 5 moles of Al in excess however because it is a solid, we disregard it since the question asked for the n(gas) present after reaction is completed.
Hence, 1.5 + 1 = 2.5 mol.

17. B is the limiting reagent. n(R2B3) = 1/3 n(B) = 20/3 ~ 6.7 ~ 6. Pretty dodgy question I have to say LOL

[Eiffel]

can someone tell me the purpose of a two way chromatogram and its advantages over a normal one?

thanks

[Eiffel]

could someone kindly please explain what PPM means (parts per millions - but what does this mean) and exact the two or three ways to calculate it (that i have seen).

Thank you

[IndefatigableLover]

can someone tell me the purpose of a two way chromatogram and its advantages over a normal one?

thanks

Increasing accuracy when determining how many components are in the sample (due to overlap or not showing up using a specific solvent as your mobile phase).

could someone kindly please explain what PPM means (parts per millions - but what does this mean) and exact the two or three ways to calculate it (that i have seen).

Thank you

PPM means that the units you'll be in will differ by a factor of 10^6. For example, 2 mg/kg is the same as 2x10^-3/2x10^3 which is a difference of 10^6 aka PPM. Calculating it is just a matter of converting units till you get it to differ by 10^6.

[paper-back]

could someone kindly please explain what PPM means (parts per millions - but what does this mean) and exact the two or three ways to calculate it (that i have seen).

Thank you

This is how I've interpreted it:

PPM means parts per million
An example of PPM is mg/L

As 1L can be written as 1KG=1000g
And 1000g can be re-written as 1000000mg
We can say that that in 1L there are 1 million mg "parts", hence the term part(s) per million

So if we have 0.1M of NaOH
0.1M can be written as 4g/L
4g/L=4000mg/L
Therefore 0.1M(NaOH)=4000PPM

[Eiffel]

i see.

This is how I've interpreted it:

PPM means parts per million
An example of PPM is mg/L

As 1L can be written as 1KG=1000g
And 1000g can be re-written as 1000000mg
We can say that that in 1L there are 1 million mg "parts", hence the term part(s) per million

So if we have 0.1M of NaOH
0.1M can be written as 4g/L
4g/L=4000mg/L
Therefore 0.1M(NaOH)=4000PPM

when you describe: 1L there are 1 million mg "parts", hence the term part(s) per million

there are a million mg parts, but parts per (what million?) id understand something like 0.5grams per litre as there are 0.5 grams in one litre, but with 1 million mg parts, it is over 1 litre, so 1 millions mg parts per litre? this is what has gotten me confused in the first place.

is there also the way of doing (grams x 10^6)/(grams of solution)

[grannysmith]

i see.

when you describe: 1L there are 1 million mg "parts", hence the term part(s) per million

there are a million mg parts, but parts per (what million?) id understand something like 0.5grams per litre as there are 0.5 grams in one litre, but with 1 million mg parts, it is over 1 litre, so 1 millions mg parts per litre? this is what has gotten me confused in the first place.

is there also the way of doing (grams x 10^6)/(grams of solution)

It's essentially a difference of 10⁶ between the 'top' base unit and the 'bottom' base unit, where the base units are grams (g) and litre (L).

[keltingmeith]

where the base units are grams (g) and litre (L).

Not necessarily, the base units can be anything really. For example, in an NMR, the chemical shift is measured in ppm, where the base unit is Hz.