VCE Chemistry Question Thread

[RazzMeTazz]

Anyone?

70 is two significant figures.
An easy way to remember significant figures is to remember that: every figure is significant except for leading 0's.

So if you wanted to write 69.767 to 2sf,  you could write either one of 70 or 7.0*10 !

[knightrider]

70 is two significant figures.
An easy way to remember significant figures is to remember that: every figure is significant except for leading 0's.

So if you wanted to write 69.767 to 2sf,  you could write either one of 70 or 7.0*10 !

Thanks RazzMeTazz 

I always thought 70 had one significant figure 

[RazzMeTazz]

Based off this graph (attached below) the question was: "Why do the differences in boiling points between alkanes and their corresponding alkanols become smaller as the number of carbon atoms become larger?"

And the answer: The proportion of hydrogen bonding to dispersion forces decreases.

But I didn't really understand this. I thought as the number of carbon atoms increases in both the alkanes and the alkanols  the dispersion forces would increase by the same amount?

Thanks

[lzxnl]

Based off this graph (attached below) the question was: "Why do the differences in boiling points between alkanes and their corresponding alkanols become smaller as the number of carbon atoms become larger?"

And the answer: The proportion of hydrogen bonding to dispersion forces decreases.

But I didn't really understand this. I thought as the number of carbon atoms increases in both the alkanes and the alkanols  the dispersion forces would increase by the same amount?

Thanks

Let's increase 1 by 10
Then increase 10000 by 10
Which increase is proportionally larger?

[knightrider]

Just wanted to clarify

Does 90 have 1 or 2 significant figures?

[Rishi97]

Just wanted to clarify

Does 90 have 1 or 2 significant figures?

2
If u are struggling with sig fig, try this tutorial its really good: http://www.chem.sc.edu/faculty/Morgan/resources/sigfigs/sigfigs3.html

[knightrider]

I was just reading some stuff on significant figures and i got confused by the (attached ).

How can those numbers have  2 or 3 significant figures and 3, 4, or 5 significant figures.?

[RazzMeTazz]

I was just reading some stuff on significant figures and i got confused by the (attached ).

How can those numbers have  2 or 3 significant figures and 3, 4, or 5 significant figures.?

Where is this from? Because I think there is a difference in the way significant figures are calculated for chemistry and other subjects like maths or physics! I'm not sure though!

[keltingmeith]

I was just reading some stuff on significant figures and i got confused by the (attached ).

How can those numbers have  2 or 3 significant figures and 3, 4, or 5 significant figures.?

The first one may have 2 or 3 significant figures because the writer has not made it clear if they are significant.

This particular excerpt is not a lesson in counting significant figures, but rather to be clear by use of a decimal point or scientific notation.

[dankfrank420]

Can someone explain what the stationary phase and the mobile phase are  and use examples from TLC to describe it?

[keltingmeith]

Can someone explain what the stationary phase and the mobile phase are  and use examples from TLC to describe it?

The "stationary phase" is the thing that doesn't move, where the "mobile phase" is the thing that doesn't move. We never consider the sample to be a phase, it is only moved by a phase.

Consider paper chromatography (simplified form of TLC), separating the colours of a texta. In this case, the texta is the sample, so we do not call it either phase. The paper doesn't move, so it is the stationary phase. The water used on the paper is what moves the texta up and separates the pigments, so the water is the mobile phase.

[dankfrank420]

The "stationary phase" is the thing that doesn't move, where the "mobile phase" is the thing that doesn't move. We never consider the sample to be a phase, it is only moved by a phase.

Consider paper chromatography (simplified form of TLC), separating the colours of a texta. In this case, the texta is the sample, so we do not call it either phase. The paper doesn't move, so it is the stationary phase. The water used on the paper is what moves the texta up and separates the pigments, so the water is the mobile phase.

Cheers.

I got confused, because I took 'phase' to in the literal sense.

[knightrider]

Where is this from? Because I think there is a difference in the way significant figures are calculated for chemistry and other subjects like maths or physics! I'm not sure though!

The first one may have 2 or 3 significant figures because the writer has not made it clear if they are significant.

This particular excerpt is not a lesson in counting significant figures, but rather to be clear by use of a decimal point or scientific notation.

Thanks EulerFan101 and RazzMeTazz 

How come in 70 there are two significant figures?
The zero doesn't really mean anything .

Like take 70.0 then this has 3 significant figures
so shouldn't just 70 have 1 significant figure?(as leading zeroes are not significant?)

[knightrider]

In response to my above post i further confirmed that 40 has 1 significant figure as i initially thought?

Refer to attached.

[wunderkind52]

In response to my above post i further confirmed that 40 has 1 significant figure as i initially thought?

Refer to attached.

it's a bit of a grey area
Consider this - write 39.9999999 to correct 2 significant figures.
And also, write 39.99999 correct to 1 significant figure.
If you really want to clarify, you can write (2s.f.) or (1s.f.) after your responses in chemistry to indicate what you've done.

That's why scientific notation is wonderful - you can easily indicate the number of significant figures.