VCE Chemistry Question Thread

[I am a unicorn]

Hello
In the VCAA Data book there is a table that gives H NMR Data showing the chemical shifts for different 'types of protons'. It has diagrams like R-CH3, etc.
What does the R stand for? And does R-CH3-R indicate that the thing on either end of the molecule must be EXACTLY the same in order to have that particular chemical shift?

Thankyou

[wunderkind52]

Are the terms 'retention factor' and 'retardation factor' in chromatography, synonymous?

Yes

[RazzMeTazz]

Are we expected to know how changes in water vapour and temperature would affect the Rf value of a particular chemical?

Thanks

[Sense]

Lead exposure can cause permanent brain damage in infants and young children, even at very low levels. The amount of lead in infant milk formula can be measured by atomic absorption spectroscopy. A 2.5 g sample of milk powder was dissolved in 50 mL of distilled water.

a   Determine the concentration of lead (ng mL–1) in the diluted milk powder solution.
2.45 ng mL -1

b   Calculate the concentration of lead (ng g–1) in the dried milk powder.
2.45 * (52.5/2.5) = 51.45 ng g-1 (My answer)

2.45 * (50/2.5) = 49 ng g-1 (Correct answer)


'A 2.5 g sample of milk powder was dissolved in 50 mL of distilled water.'
Wouldn't this mean that the whole solution is 52.5 mL? Not 50mL?

[keltingmeith]

Lead exposure can cause permanent brain damage in infants and young children, even at very low levels. The amount of lead in infant milk formula can be measured by atomic absorption spectroscopy. A 2.5 g sample of milk powder was dissolved in 50 mL of distilled water.

a   Determine the concentration of lead (ng mL–1) in the diluted milk powder solution.
2.45 ng mL -1

b   Calculate the concentration of lead (ng g–1) in the dried milk powder.
2.45 * (52.5/2.5) = 51.45 ng g-1 (My answer)

2.45 * (50/2.5) = 49 ng g-1 (Correct answer)


'A 2.5 g sample of milk powder was dissolved in 50 mL of distilled water.'
Wouldn't this mean that the whole solution is 52.5 mL? Not 50mL?

Definitely not. 1g does not increase volume by 1 mL, this is dependent on the density of the material. In fact, if dissolved properly, the full 50 mL will not be added until the powder is properly dissolved, so as to prevent this error. This way, if we dissolve the compound in 50 mL, we know we can treat the volume as 50 mL.

[Rishi97]

Hey guys.. i know im in uni and this is the 3/4 thread but I need help with a q that we have defs covered in yr 12. I'm just having a major mental blank at the moment haha...

Give the pH and pOH of :
a) a solution prepared by adding 25.0ml of 0.0200 M NaOH to 50.0 mL of 0.00800 M HCl

Thanks !!!!

[Sense]

In a particular experiment, 12.5g of a sample of steel containing manganese oxidised to permanganate, The volume of the solution was made up to 100 mL. The absorbance of a 1 mm thick sample of this solution was then measured in a colorimeter to be 0.65 at a preselected wave length of 600nm.

A set of standard solution of potassium permanganate were made up and the absorbance of 1 mm thick samples of these solution were measured. A graph of permanganate ion concentration versus absorbance is given below.

Q1) What is the concentration of permanganate ions in the solution whose absorbance was 0.65?
Answer: 0.20M

Q2) Calculate the mass of manganese in the steel sample?
1.1?

'The volume of the solution was made up to 100 mL' means that it was diluted, right? I'm not sure because it usually states if it has been diluted or water has been added.

[Sense]

Definitely not. 1g does not increase volume by 1 mL, this is dependent on the density of the material. In fact, if dissolved properly, the full 50 mL will not be added until the powder is properly dissolved, so as to prevent this error. This way, if we dissolve the compound in 50 mL, we know we can treat the volume as 50 mL.

Thanks!

[cosine]

Why can't we use the name 3-propanol?

[Mieow]

Because then it would just be 1-Propanol but mirrored

[cosine]

Thanks mate.

How can I name this hydrocarbon compound?

(CH3)2CHCH2CH3

[wunderkind52]

Thanks mate.

How can I name this hydrocarbon compound?

(CH3)2CHCH2CH3

Off the top of my head I think that's 2-methylbutane. Try drawing the structural formula, semistructural can be confusing to read at times!

[cosine]

Off the top of my head I think that's 2-methylbutane. Try drawing the structural formula, semistructural can be confusing to read at times!

I cant seem to draw the first part (CH3)2

[wobblywobbly]

I cant seem to draw the first part (CH3)2

(CH3)2CHCH2CH3

Two CH3 groups branch off the CH.

[IndefatigableLover]

Hey guys.. i know im in uni and this is the 3/4 thread but I need help with a q that we have defs covered in yr 12. I'm just having a major mental blank at the moment haha...

Give the pH and pOH of :
a) a solution prepared by adding 25.0ml of 0.0200 M NaOH to 50.0 mL of 0.00800 M HCl

Thanks !!!!

n(NaOH) = CV
= 0.0200 x 0.025
= 0.0005 mol
n(OH-)= 0.0005 mol

n(HCl) = CV
= 0.00800 x 0.05
= 0.0004 mol
n(H+) = 0.0004 mol

We look at which one we have most in excess (which happens to be OH-).

Therefore:
n_Excess(OH-) = 0.0005 - 0.0004
= 0.0001 mol

The total volume is 75mL. Therefore we can work out the total concentration of the mixture of NaOH and HCl.

C(OH-) = n/v
=0.0001/0.075
= 0.00133

pOH = -log10([OH-])
= 2.875

pH = 14 - pOH
= 11.1249

For your final answer, it should be to 3 significant figures so:

pOH = 2.88
pH = 11.1