AP Chem Sig Figs

Test Preparation
1

<p>When using a periodic table to find the molar mass of a compound, do the significant figure rules apply? For example: would the molar mass of CO2 be 44.01(12.01+32) g or 44? would the molar mass for H2O be 18.02(2.02+16) or 18? (all numbers from my periodic table). Which would be best suited for the test? and if your teacher wants you to use significant figures for calculations, do u believe they apply when finding the molar mass?</p>

<p>*Also, would it matter if I added 12.01 + 16.00 to get 44.01 in sig figs....?</p>

2

<p>Every periodic table I've used has all molar masses listed to at least the hundredths place. In which case, it would be 12.01+32.00=44.01</p>

<p>Does yours actually round some elements to whole numbers and not others?</p>

3

<p>As I understand it, addition doesn't apply to sig figs the same way you're talking about.</p>

<p>Also, the masses are all usually calculated to the same fraction (in Zumdahl they're all to the nearest hundredth, except hydrogen is 1.008). Therefore, I'd assume 16.00 for oxygen, even though it says 16.</p>

4

<p>it is usually very beneficial to have accurate molar masses for the compounds. . .it sounds like your table rounds the mass for oxygen from 15.999 to 16, which is very abnormal. . .
so to answer you question, you definitely need to apply the rules when finding molar mass. . .you table is a little weird, so I would ask your teacher for a copy of the table they give you on the AP test</p>

5

<p>the way you did it is right. . .NEVER assume that 16 is 16.00 or something on the AP test. . .I think the only thing wrong with your calculation is a bad periodic table</p>

6

<p>I think you can be +- one sig fig on the AP test and still have it be right. The way our teacher taught us was to always use three if you get confused.</p>

<p>:)</p>

7

<p>On an AP test, you generally don't show calculations for molar mass. And if I remember correctly, the table only goes out 2 places.</p>

8

<p>In HS chem, oxygen is almost always taken as 16.00. It would take a compound with many oxygen atoms for there to be a different in calculation using 15.9994 and 16.00</p>

9

<p>Well if it helps, and my teacher isn't so great by AP teacher standards, but if it helps he says that it's ok to just round to whole numbers or tenths digits...like oxygen is 16 and nitrogen is 12 etc. etc.</p>

10

<p>i kind of forgot to do sig figs on the exam....and i still got a 5</p>

11

<p>I thought N was 14.01... that's some interesting rounding method he uses to get 12.</p>

12

<p>ohh fine sorry i haven't taken chem for 2 yrs so i switched carbon and nitrogen, sue me</p>