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Dealing with Significant figures is one of the most confusing things in chemistry, or
science in general. This video will show you how to add or subtract
using significant figures as well as a shortcut you can take to make your life easier.
Here is a multiple choice question from a chemistry regents exam asking you to add up
4 numbers and then select the answer that expresses the sum to the correct number of
significant figures. There are 3 simple rules to follow when adding
and subtracting with significant figures, 2 of which your calculator typically does
for you and they are: Lining up the decimal points, which, as you'll see in a few seconds
is the most important step. The second step is to add or subtract, depending
on the sign. The 3rd step is to ensure that the sum or
difference has the same number of digits AFTER the decimal point as the measurement with
the FEWEST digits AFTER the decimal point. This is why lining up the decimal points is
important. Here's what this means: the first measurement
of 6.6412 has a total of 4 digits after the decimal point, meanwhile the 2nd measurement
only has 2, which is currently the FEWEST. The 3rd has 3 and the last measurement also
has 2. Therefore, the sum, can only have 2 digits
after the decimal point which means we will need to refer to the next digit to see whether
or not to round up or leave it alone. Since the next digit is a 5 or greater, we'll
round the prior digit up and leave off any of the remaining digits, making the answer
choice 4. But here's a tiny little shortcut when it
comes to adding or subtracting with significant figures.
As long as you know to count the number of digits AFTER the decimal point, the correct
answer will be the one that has the same number of digits AFTER the decimal point as the measurement
with the FEWEST, which in this case is 2. The only time this wouldn't work is if there
was more than 1 choice that also had the FEWEST number of digits after the decimal; in that
case you will need to calculate and round correctly.
This shortcut also works for subtraction problems. Without calculating, you should now be able
to quickly tell what the answer is! Before I show you the correct answer, please
take a moment to subscribe to my channel so you can be the first to know when a new video
gets produced! Okay, so the correct answer is... choice 2
because it is the only choice that has 1 digit after the decimal point just like the measurement
with the fewest digits after the decimal point. If you found this video helpful, please let
me know by giving it a thumbs up or leave your feedback in the comments section below
this video.