Question

Question: Solve: 3.12g + 0.8g + 1.033g (with significant figures) Answer: 5.0 (Look at the image below: Why is C Correct?) WHY IS THIS RIGHT? i thought it was A?

Why isn't the answer 5 and why is it 5.0?5.0 has two significant figures and when you add sig figs you need to lowest number of sig figs from the problem and the lowest number of sig figs is 1 since 0.8 has only 1 sig fig. So shouldn't the answer be 5? Why is it 5.0? (2 sig figs?)

Answer 1

The correct answer is C) 5.0 g.

Explanation:

The significant figure rules are different for addition and subtraction than in multiplication and division.

For addition and subtraction, the answer can contain no more digits past the decimal point than the number with the fewest digits past the decimal point.

Here is what you do:

1. Add or subtract in the normal fashion.

2. Count the number of digits in the decimal portion of each number

3. Round the answer to the FEWEST number of places after the decimal point for any number in the problem.

Thus, we get

`color(white)(m)3.18color(white)(mml) "g"`
`+ 0.8color(red)(|)color(white)(mll) "g"`
`ul(+1.033color(white)(mll) "g")`
`color(white)(m)5.0color(red)(|)13color(white)(ll)"g"`

The sum is 5.013 g.

Now you round off to the appropriate number of decimal places.

The number 0.8 has only digit after the decimal point, so you must round off the answer to one digit after decimal point (at the red bar).

The first digit past the red bar is 1, so you drop all numbers after the red bar and leave the remaining digits unchanged.

The correct answer becomes 5.0 g.