Significant Figures

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Significant Figures

Postby Michael_Johanis_2K » Wed Feb 01, 2017 5:56 pm

Hi. This question is for TA Jack, who read my email and said he would answer this:

When we use bond enthalpy, reactant - product, and Hess's Law, assuming that we have exact numbers of moles (from the equation), the significant figures in the answer are ONLY limited by addition, right?

For instance: 25 kJ/mol (2 mol) - 3274 kJ/mol * 1 mol = -3224 kJ
There is no rounding to two significant figures (the 25kJ/mol does NOT limit the answer to 2 sig fig). It stays at 4 because every number is significant *until the ones digit*, right? The way I see this is that it is 50. KJ - 3274 kJ, because I think that the multiplication does not affect significant figures.

Thank you.

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Re: Significant Figures

Postby Chem_Mod » Wed Feb 01, 2017 6:21 pm

When combining the operations of multiplication/division and addition/subtraction, the significant figures must be determined separately. Your example is correct, but the multiplication did affect the significant figures (25 kJ x 2 = 50. kJ, intermediate answer has 2 sig figs).
For example: 25 kJ/mol (4 mol) - 3274 kJ/mol * 1 mol = -3170 kJ
This answer has three sig figs because the first multiplication results in 1.0 x 102 kJ, which is only significant to the tens place. The original number could have been anywhere in between 24.6 and 25.4, so after multiplication it could be between 98.4 and 101.6, and the final answer could be between -3175.6 and -3172.4 -- the ones place is now uncertain, and the answer is only significant to the tens place.

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