Hi all,
I'm a bit confused with Significant figure marks. Is it true you only lose one mark for a sig fig mistake on the exam? Also, if a question, for example, gave data with sig figs of 2 but then a subsequent question like 1b or 1c uses sig figs of 3 in that question, should I be using Sig figs of 2 or 3 in my answer?
This has been answered loads of times, so please forgive me for not going into full detail - we don't know that it's only one mark per paper, but it's highly likely that it is. VCAA themselves don't say it publicly, but most high school teachers do, and teachers do have private channels that we can't access.
As for the other question - you always use the smallest number of sig figs in your calculation. Example, question 1 is - "you have 1.0 M hydrochloric acid and neutralise 2.0L of it with 5L of sodium hydroxide"
1.a) How much HCl, in mol, is there?
This would be calculated as n=1.0*2.0=2.0. Even though the smallest number of sig figs in the question is 1 (the 5L), the smallest number of sig figs in THIS calculation is 2 (the 1.0 and the 2.0), and so you round the answer to 2.0.
1. b) What's the concentration of the NaOH in mol/L?
This time, you'd calculate as C=2.0/5=0.4 M. In this case, the smallest number of sig figs you used is 1, so you leave it at one sig fig.
Another example - question 2 is, "50.0kg of soil is treated with acid to remove metal contaminants. After treatment with excess acid and allowing the soil to dry, the soil now weighs 7 kg"
2.a) How much of the soil, in kg, was metal contaminants?
Okay, so to calculate this, the metal contaminants is whatever was gone - so that would 50.0-7=43 kg. Remember that addition/subtraction follow slightly different sig fig rules, and so we do it to the smallest number of decimal places.
2.b) After the acid is treated, it is found that all of the metal waste is copper. If all of the mass of the waste found in 2.a) is converted into solid copper, how much copper, in mol, do we have?
This one is calculated as (43*1000)/63.546=6.8*10^2 mol. In this case, we have two data points - one is two sig figs (the 43) and the other is 5 sig figs (the 63.546). So, we round to two sig figs. 43 is not in the initial set of data - and that's fine. You don't use the 50.0 or the 7 in this calculation, so they should not affect the sig figs in rounding. In fact, if we did, we'd have to round to a different number, because they have 3 and 1 significant figures (respectively). But since the 43 has two, and the 43 is what we used in the calculation, that's the one we pay attention to
Hopefully that all makes sense and clears up any confusion - ignore the sig figs being used in the question, and just focus on the amount of sig figs in the calculation - THOSE are the ones you use
Whats the difference between the validity and the limitations of an experiment?
Validity = is experiment telling you what you want to know. An example of an invalid experiment is eating grass while blindfolded to find out what colour the sky is. An example of a valid experiment is looking at the sky to find out what colour the sky is
Limitations = issues with your experiment that might mean you're not getting the full picture. A limitation of the previous experiment is what if you have vision problems, or what if the colour of the sky changes with time. To fix these limitations, you might ask other people what colour they'd call the sky instead of just observing it yourself, and you might ask at various times in the day/night