General Rounding Question

[Emily Vainberg 1D]

When you are finding the molarity of a solution, do you always use the full value on the periodic table or can we round to three decimal points?
Ex. if there are 15 grams of Hydrogen would you do (15/1.00974) to find the amount of moles?

[KeyaV1C]

I’m pretty sure it’s okay to round off to 3 decimal points or fewer. I usually just use 1.01 for the molar mass of hydrogen.

[sbeall_1C]

I agree! I usually use 2 or 3 decimal places off the molar mass of the periodic table. Using more decimal places will result in a more accurate answer, but make sure you pay attention to significant figures for your final answer!

[Megan Cao 1I]

you can do whichever you prefer. since the value's a constant the only time you need to worry about sig figs is when you're calculating your answer.

15g H * (1 mole/1.00974g H) = 14.85530929 moles
since 15 has 2 sig figs your answer would be 15

15g H * (1mole/1.01g H) =14.851485 moles
with 2 sig figs it'd still be 15

[Caitlyn Tran 2E]

I asked Professor Lavelle this question after the first class. He told me that we should use the full value on whatever periodic table we are given, so if he gives us a periodic table that goes out 3 decimal places, write the entire decimal. I believe this is just to prevent rounding errors since we are technically not supposed to round according to significant figures until the end. For homework, however, I am not sure what the expectations are.

[505306205]

Try to avoid rounding until the very end when you have to give the answer with the correct number of significant figures, which is the least number of significant figures among the numbers given in the question.

[Ashley Kao 1H]

I think that rounding to two or three places after the decimal place should be accurate enough. I remember doing a problem that involved finding the molarity of Hydrogen and we utilized 1.01 as its molar mass.

[Maya Pakulski 1D]

Throughout the problem is it safe to carry three decimal places through all your math and then round to sig figs at the end or should you go out another decimal place?

[Ruby Richter 2L]

You should probably use as many decimal places as possible when you're working through the problem in order to get the most accurate result. Sometimes the answer might be off by a tenth or so depending on earlier rounding.

[Avnita C_4B]

For using the molar mass of any element, you would usually go for 2-3 sig. figures but when you write your answer, it must match up to the correct amount of sig. figures given in the question. For something like Hydrogen, the amount mostly correlates to 1g/mol.