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### Which mass of Hydrogen should I use?

Posted: Fri Apr 06, 2018 1:28 pm
Throughout section E's exercises in the solution manual, the mass of Hydrogen is approximated to be 1.01 g/mol. While in section F of the solution manual, the mass of Hydrogen is approximated to be 1.0079 g/mol. Using 1.01 g/mol changed my answer by a hundredth in a couple problems. Is there a reason why the empirical formula section is using a more precise value of the mass of Hydrogen? Hydrogen was the only element in this section that changed. In the future, if my answer is a couple hundredth off of the correct solution because I used 1.01 instead of 1.0079 will I lose points?

### Re: Which mass of Hydrogen should I use?

Posted: Fri Apr 06, 2018 1:33 pm
To be safe, you can use the more specific mass - I always just use 1.008 which is the value most periodic tables have, and uses the same number of sig figs that are other atomic masses are given in (ex. 12.011 for C, 15.999 for O)
In the modules, the atomic mass has been given a lot of the time, so the atomic mass of H would potentially be given in a problem. If our answers are off by a hundredth, I don't think points will get taken off, because we still know how to do the problem. He said in lecture the other day that he likes to give partial credit when there's partial credit to give, so if we're off by a hundredth we'll hopefully be good!

### Re: Which mass of Hydrogen should I use?

Posted: Fri Apr 06, 2018 1:36 pm
I believe the last digit in the numerical value is an "estimate", so I don't believe you would lost points if the answer you proposed is off in the hundredths place.

### Re: Which mass of Hydrogen should I use?

Posted: Fri Apr 06, 2018 1:37 pm
My TA said that sig figs wouldn't cause a loss in points this quarter. She said in the past students had to watch them and then people ended up focusing more on sig figs and rounding correctly than doing the problem right in the amount of time given. She recommends if you are using a calculator keep as many decimals as possible (or like 3 or 4) and then at the end you can round and they won't be taking off points for slight variations if they see you are doing the right thing and it came down to a difference in decimals somewhere along the way. You should confirm and check with your individual TA but this is what mine said. So for hydrogen I've been using 1.008 because thats what most periodic table charts go to and 3 decimals is plenty, and then in the end I like to keep 3 decimal places for my answer.