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If my empirical formula is CH2, for example, and then I need to use the molar mass to find the molecular formula, I do 1(12.011 g.mol^-1)+2(1.008 g. mol^-1). Will my answer have 3 places after the decimal because I am supposed to focus on the addition rule or will it be 4 significant digits because of the multiplication rule?
You only multiply when you're taking into account how many carbons and hydrogens are in the empirical formula. Since your final operation is addition, you would focus on the significant figure rules for addition. In this case, you would limit your answer to 3 decimal places.
In your first multiplication calculation (1*12.011), you will end up with 5 significant figures. In your second multiplication calculation, (2*1.008), you will end up with 4 significant figures. However, in your last calculation, you adhere to the addition/subtraction rule for significant figures.When you add (12.011+2.106), you will end up with 14.027, which has 5 significant figures, but this is fine because the addition/subtraction rule states that you round to the precision of the least precise addend which refers to the thousandths place. You don't have to take into account the extra significant figure before the decimal place in the addition/subtraction rule.
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