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May 27, 2015

Posted by **Avonlea** on Monday, October 29, 2007 at 4:56pm.

- chemistry -
**DrBob222**, Monday, October 29, 2007 at 5:03pm134 mm.

1 mm = 1 torr

760 mm = 1 atm

101,325 Pa = 1 atm.

Here is a site on significant figures. http://dbhs.wvusd.k12.ca.us/webdocs/SigFigs/SigFigs.html

- chemistry -
**Avonlea**, Monday, October 29, 2007 at 5:34pmeverything I tried was wrong for the conversions and the sig figs still don't make sense. How do you make a big sig fig into a smaller one using 8339? Please give an example.

- chemistry -
**DrBob222**, Monday, October 29, 2007 at 6:11pm8339 to four sig fig is 8339. (includes the decimal to show that the 9 is signifigant) OR

8.339 x 10^3.

To 3 s.f. we round the last 39 to 40 and write 8.34 x 10^3 (you may NOT include the 0 from the 40 for that would show 4 places).

To 2 s.f. we drop the 4 and it becomes 8.3 x 10^3.

To 1 s.f. we drop the 3 and it becomes 8 x 10^3.

Check my work. I will go through a conversion on a separate post.

- chemistry -
**DrBob222**, Monday, October 29, 2007 at 6:24pmFor a conversion from mm to atm.

There are TWO conversion factor, in reality, but both really are the same thing; one is just the reciprocal of the other.

760 mm = 1 atm so we may write it one of two ways.

760 mm/1 atm OR 1 atm/760 mm.

Now suppose we want to convert 340 mm to atm. We take WHAT WE ARE GIVEN and multiply it by a conversion factor. So we have 340 mm x factor = ?? atm.

All we need to do is to supply the factor. We can write it now one of two ways. We may write

340 mm x 1 atm/760 mm = ?? atm

OR

we may write 340 mm x 760 mm/1 atm = ?? atm.

Those are the only two choices you have. You can GUESS at the answer and be right half the time or you can do it logically and be right all the time. We simply look at the units. We ALWAYS want the unit we don't want to cancel and we want the unit to which we are converting to stay. That means, for this problem, that we want mm to cancel and atm to stay. Analyze the two answers I wrote above. The first one has mm in the numerator of the first term and mm in the denominator of the second term so mm cancels. What is left? Just atm in the numerator of the second term and that's what we wanted. So that is the correct one. However, let's look at the second one just to complete the analysis.

We have 340 mm x 760 mm/1 atm = ??.

The units in the numerator are mm x mm = mm^{2}and the unit in the denominator is atm so the answer comes out in units of mm^{2}/atm and that ISN'T what we wanted. If you go through an analysis like this for each conversion you can't make a mistake. Let me know if you have any questions.