Accuracy and rounding off calculations. (1 Viewer)

[Ioup]

This applies for Physics and Chemistry I spose:

With the HSC exam do calculations have to be spot on and rounded off perfectly to score the mark?

Or is there a certain range of acceptable answers that scorers award the marks for?

I'm unsure on how to round off properly with significant figures.
If you could please post rules on how to do the process correctly with relevant worked examples.
Thanks.

 

[Dreamerish*~]

Generally, you go by the numbers in the question. If the values given in the question were rounded of to one decimal place, you follow suit and round your answer off to one decimal place. If no values were given, you should be fine rounding off to two decimal places. Read questions carefully as some specify the number of decimal places or significant figures required in your answer.

Whatever you do, your answer should never look like anything this: 0.67435632 g. From my experience, excessive decimal places = wrong answer.

There is a specific rule regarding pH and [H₃O⁺]. The number of decimal places in the value of pH should equal the number of significant figures in the value of [H₃O⁺].

 

[Riviet]

Whatever you do, your answer should never look like anything this: 0.67435632 g. From my experience, excessive decimal places = wrong answer.

The reason for this is quite simple and logical.
If you are given data that is correct to 2dp, and your answer on the calculator is say, 0.1234567, you MUST NOT write the answer as 0.1234567.
Why?
Your numerical answer can never be more accurate than the data given.

Examples of counting/identifying number of sig figs:

Just count the number of digits that you see:
7- 1 sig fig
14- 2 sig fig
196- 3 sig fig
1.2- 2 sig fig
1.02- 3 sig fig
16.53- 4 sig fig

To round something to n sig figs from a long decimal displayed on your calculator screen, I will use a couple of examples, which you should be able to see how it works.

Round 1.32123999 correct to 3 sig figs.

We count the first three digits from the left, so the answer is simply 1.32.

Round 2.86761987 correct to 4 sig figs.

We count the first 4 digits from the left, which are 2.867. However, we observe that the next digit is a 6. The rule is if the next digit is greater than or equal to 5, we must increase the last digit of the rounded number by 1, which would be the 7 in 2.867. Therefore the answer is 2.868.

I hope that helps.

 

[Ioup]

Yeah thanks.

Im still unsure as to how close my rounding has to be to score marks though.
And with Combustion questions do I leave the rounding off till the end or do it with each individual step.

 

[Dreamerish*~]

Yeah thanks.

Im still unsure as to how close my rounding has to be to score marks though.
And with Combustion questions do I leave the rounding off till the end or do it with each individual step.

Never round off during calculations.

In most cases you would round off to two decimal places. Use the memory function in your calculator, don't round off before you get the final answer, and you should be fine.

 

[pLuvia]

But if there is 2 parts to the question and one part says to round it off to 2dp then you should use that rounded off answer for the next part

 

[A l]

Too bad we can't use exact values (where applicable) in Science, like in Mathematics.