Rounding [ENDORSED]

[Emily Mendez 4C]

In all the empirical & molecular formulas questions in the homework, am I allowed to round? I see that if I round for example the moles of a certain element I get another answer than when I don't round. Can someone just notify me when its okay to round and when I shouldn't round.

[Alex Tchekanov Dis 2k]

What I was taught was that in chemistry you always want to make sure you are using the proper sig fig rules and then don't round your answer until the very end of the problem. Waiting to round to the end will prevent the slight rounding difference from each step from adding up into the wrong answer. Hopefully, that helps.

[Jainam Shah 4I]

With regards to empirical and molecular formulas, you may round the mole ratio you obtain when you take the number of moles of one element and divide it by the smallest number of moles of another element you have. The goal isn't to round but to get a whole number in the mole ratio. For example if one of your element's mole ratio divides out to be 2.96 you may round to 3. If you get a ratio such as 1.33 then you would take all the ratios and multiply them by 3, because 1.33*3 is 4, a whole number. The concept is not of what to round but rather how to get the whole number. If you are within 0.1 of a whole number I think you can round to the nearest whole number. For other decimal values try to multiply by a common factor to get all the ratios to be a whole number.

[Natalie Nartz 4F]

Refrain from rounding values within the problem it self when you are doing calculations. From my understanding, it is only appropriate to round to the correct number of significant figures when you have reached your final answer.

[Ashley Nguyen 2L]

Rounding in chemistry usually occurs at the very end of your calculations in order to prevent rounding differences from accumulating.

[405335722]

Hello! I usually round with the consideration of sig figs. So, when multiplying or dividing, you always round to the least specific place (ex. 10.0/1 would just have one sig fig because "1" is the least specific). I usually round according to sig figs during the entire problem so I get in the habit of it and never forget when submitting my final answer.

[Cynthia Rodas 4H]

I just looked over Dr. Lavelle’s worksheet regarding significant figures. According to Lavelle, one of the rules for rounding off is to wait until the end of a calculation. For calculations using multiplication and division, your answer should have the same number of significant figures as the value with the least number of significant figures. For calculations using addition and subtraction, the answer should have the same number of decimal places as the smallest number of decimal places in the problem.

[kim 2I]

Another thing about rounding at the end of your calculation is that is your last digit is 5, make sure you round to the nearest even number. For example:

If you need only 2 sig figs and your answer is 1.35, then your answer would be 1.4. If you had 1.65, then your answer would be 1.6.
*Remember that this rule only applies when there is no number that follows the "5."

[Maika Ngoie 1B]

Rounding can be a bit tricky and if possible, I would keep the rounding to a minimum while you work out the problem. However, if you do, make sure to use the proper sig figs and your answer should be okay.

[Victoria Otuya 4F]

I keep on seeing sig figs throughout this discussion. I am not knowledgeable about that term. Can someone explain to me what it means and how it helps with rounding?

[Eesha Sohail 1D]

I keep on seeing sig figs throughout this discussion. I am not knowledgeable about that term. Can someone explain to me what it means and how it helps with rounding?

"Sig figs" is a shorthand term for "significant figures," a term in chemistry used to help gauge level of accuracy as well as error. The general rule is that your answer should contain the same number of significant figures as the term in the problem with the lowest number.

Example: Your calculations for a random problem yield an answer of 3.1415, and the question contains the values 7.29 and 34.5. Both these values have three "significant figures," which are the digits that "actually matter," in a sense. Given that level of accuracy, our answer is too specific (3.1415 = 5 sig figs) and doesn't take into account the possible error. So, we round the answer to three sig figs: 3.14.

Rules for calculating sig figs:

1. Nonzero digits are ALWAYS significant.
2. Zeroes in front of the number (e.g. in 034.45, or 0.27, or 0.00065) are NOT significant.
3. Zeroes inside the digits (captive zeroes, e.g. 305.04) are ALWAYS significant.
4. Trailing zeroes without a decimal point (e.g. 35000) are NOT significant.
5. Trailing zeroes WITH a decimal point (e.g. 670.) are ALWAYS significant.
6. Zeroes after the decimal point (e.g. the last zero in 0.910) are ALWAYS significant.

If I missed anything, feel free to let me know!

[Johnathan Smith 1D]

Thank you for this!! I had little knowledge on sig figs, but this is really helpful!!!

[Juana Abana 1G]

"Sig figs" is a shorthand term for "significant figures," a term in chemistry used to help gauge level of accuracy as well as error. The general rule is that your answer should contain the same number of significant figures as the term in the problem with the lowest number.

Example: Your calculations for a random problem yield an answer of 3.1415, and the question contains the values 7.29 and 34.5. Both these values have three "significant figures," which are the digits that "actually matter," in a sense. Given that level of accuracy, our answer is too specific (3.1415 = 5 sig figs) and doesn't take into account the possible error. So, we round the answer to three sig figs: 3.14.

Rules for calculating sig figs:

1. Nonzero digits are ALWAYS significant.
2. Zeroes in front of the number (e.g. in 034.45, or 0.27, or 0.00065) are NOT significant.
3. Zeroes inside the digits (captive zeroes, e.g. 305.04) are ALWAYS significant.
4. Trailing zeroes without a decimal point (e.g. 35000) are NOT significant.
5. Trailing zeroes WITH a decimal point (e.g. 670.) are ALWAYS significant.
6. Zeroes after the decimal point (e.g. the last zero in 0.910) are ALWAYS significant.

If I missed anything, feel free to let me know!

Thank you so much this is really really helpful. Now I have a better understanding of what significant figures are.