Sig Figs

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Mariam Baghdasaryan 4F
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Joined: Fri Sep 28, 2018 12:29 am

Sig Figs

Postby Mariam Baghdasaryan 4F » Thu Oct 04, 2018 10:49 pm

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?

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Re: Sig Figs

Postby Chem_Mod » Thu Oct 04, 2018 11:02 pm

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.

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Re: Sig Figs

Postby Luis_Yepez_1F » Thu Oct 04, 2018 11:02 pm

If I remember correctly, when it comes to a combination of both multiplication/division and addition/subtraction. You would adhere to the addition rule for sig figs.

Matthew Choi 2H
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Re: Sig Figs

Postby Matthew Choi 2H » Fri Oct 05, 2018 3:22 am

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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