Significant figures Guide

[kenhung123]

It all falls down to how VCAA does it though.

[stonecold]

I was always confused about significant figures right up until the week before my Unit 3 exam. Then it became so clear and easy to do

A comment from someone who did so well warms my heart a little.

And just to clarify, you don't allow for relative atomic mass when determining sig figs yeah?

[kyzoo]

Hmm I think relative atomic mass is to 1 decimal place, so if it can be 2, 3, 4 or 5 signifia cnt figures dependingo n the number of digits needed

[stonecold]

So if i've got 20.0 g hydrogen gas, and convert to mol, is it 10 g mol or 10.0 g mol ?

[kyzoo]

M(H2) = 2.0 g/mol
m(H2) = 20.0g

So n (H2) = m/M = /20.0/2.0 = 10 mol

[stonecold]

other way around kyzoo :p

[Michael0007]

I thought we didn't need to take into account the sig figs of the periodic table?

could someone please clarify point no.2 of the Vcaa report aswell?

[brainbloopers]

could someone please clarify point no.2 of the Vcaa report aswell?

Point 2 is confusing basically they are trying to say that with multiplication you always use the lowest sig fig but with addition and subtraction it's the lowest decimal point in the whole calculation.

2 (1s.f) *12.0 (3 s.f) = 20 1.sf (very unintuitive I know ! )  + 1 (1 s.f) * 6.0 (2 significant figures) = 6 1 s.f + 2 (1 s.f) * 16.0 (3 s.f) =  20 (1.sf) then 20+6+20 = 46 BUT SINCE YOU ADDED you use the lowest decimal place number in the calculation which was one therefore it's 46.0

Hope that helped.

[Reckoner]

Say you have to calculate the amount of charge using Q=It, and you get 5.175C (exact), but to 3 s.f it goes to 5.18C.

If you then work out the n(e-), and your write
n(e-)= 5.18/96500 
=5.36 x 10^-5   (using the "exact" 5.175 from the calculator)

Is that ok? because the value of 5.18/96500 is 5.37 x 10^-5.

[k0k0p0p]

I have been taught with significant figures to round only with the final answer. For all calculations you keep the numbers in your calculator and use the 'ans' button for consequential calculations. This avoids errors due to using rounded figures.
When writing in the middle of a calculation, you can write down a few more figures, for example if the question calls for 3 significant figures then write 3.14152 or something similar. You could write 3.14 for a step in a 3 significant figure question, but keep the numbers in your calculator. I would NEVER write a calculation with fewer figures than are called for in the answer, ie. do not write 3.1 for the above value.
With the final figure, ensure significant figures are perfect. Convert to scientific notation if there could be any dispute over the number of significant figures, ie. 0.00314 would be 3.14 *10^-3 to 3 significant figures.
For pH, the numbers after the decimal point are significant, ie. 3.14 would be to 2 significant figures (not 3).
There will always be at least one mark that depends on using the correct significant figures within the exam, but this could be any of the questions so ensure all answers use the correct significant figures.
Essentially, keep the figures in your calculator and use these between calculations. Also, be careful with the acidity constants in the data book as these are only to 2 significant figures! (gotta love pi!)

[Shyam995]

Stuck. Because it is quite significant

lol love the pun

[NathanJ]

Ok, what happens if in the question everything is given in 4 sig-digs, does that always mean that the answer will be in 4 or can the working out change it?

eg.
A student determined the water content of a sample of jam. The following
measurements were obtained:
Mass of evaporating dish: 20.22 g
Mass of jam and evaporating dish
before heating: 30.95 g
after heating: 27.22 g
after more heating: 26.50 g
after more heating: 26.49 g
What was the percentage, by mass, of water in the jam?
A5.
Step 1 Find the mass of the moist jam by subtracting mass of evaporating dish.
Mass of moist jam = 30.95 – 20.22 g
= 10.73 g
Step 2 Find the mass of water.
Mass of water = 30.95 – 26.49 g
= 4.46 g
Step 3 Find the percentage of water in moist jam.
% (H2O) =
10.73
4.46 × 100
= 41.57%
Step 4 Express the answer with the correct number of significant figures. (this is where my question comes into play.)

Is the % (H2O) = 41.6% or does it equal 41.57% because of the 4 sig digs in the question ignoring the 3 that happens during the working out.

Cheers,