6th Edition 11.61

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Nathan Tran 4K
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Joined: Fri Sep 28, 2018 12:16 am

6th Edition 11.61

Postby Nathan Tran 4K » Tue Jan 22, 2019 11:30 am

I have a question regarding sig figs. In 11.61, all the initial concentrations are given in three sig figs, but the Kc is given in two sig figs (the value is 0.56). The question asks to find the equilibrium concentrations of all the reactants and products. Why does the answer manual have the answers in three sig figs when the Kc only has two sig figs?

Cody Do 2F
Posts: 62
Joined: Fri Sep 28, 2018 12:23 am

Re: 6th Edition 11.61

Postby Cody Do 2F » Tue Jan 22, 2019 12:46 pm

The reason why the answers are given in three sig figs instead of two is due to the rules of significant figures. When you do the ICE table to find the x value, the x-value is in fact given to two sig figs due to the Kc's value of 0.56 (x is equal to 0.07). However, as you're asked to find the equilibrium concentration, you have to then add/subtract this value from the initial values. When you add or subtract two values, the rule is that you round off at the smallest decimal place of the values used.

Thus, since the initial values (3.00 mol PCl5, 6.00 mol PCl3, 1.00 mol Cl2) are added/subtracted by 0.07. Since all numbers are to the hundredth place, the answer will be to the hundredth place and thus have three sig figs.

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