1. What is the mass of 2.00 mol NH3? A. 20.0 g, B. 34.1 g, C. 30.0 g
QUESTION:
What is the mass of 2.00 mol NH3?
A g, B g, C g
Molar mass of NH3: g/mol
mass NH3
17.03 g NH3 =
moles NH3
1 mol NH3
What is the mass of 2.00 mol NH3?
A g, B g, C g
PAUSE
By now, you should know how to calculate the molar mass given a formula.
CLICK For NH3, you should be able to calculate that the molar mass is grams per mole.
To do a mole-to-mass conversion, it is important to recognize that these two quantities are directly proportional.
This means that the ratio of the two quantities is constant.
In this case,
The ratio of the mass of ammonia
CLICK
to the number of moles
CLICK CLICK
is equal to
the ratio of grams
to one mole
We can rearrange this equation to solve for the mass of ammonia. Essentially, what we would end up doing is
multiply the given moles of ammonia, 2.00 moles
CLICK by the ratio grams ammonia over 1 mole ammonia
CLICK CLICK CLICK
The ratio serves as a conversion factor.
SHOW CANCELLATION OF UNITS
The answer we get should be rounded off to three significant figures... since the the least precise
factor, 2.00
CALLOUT “3 significant figures”
has only three significant figures grams has four significant figures,
CALLOUT and the number 1 in the denominator is exact.
Two times gives us 34.06, which we round off to 34.1.
Therefore, the correct answer is choice B grams.
Here’s another way of looking at this calculation.
CLICK if we want to convert moles to grams...
we simply multiply the number of moles
HIGHILIGHT
by the molar mass
HIGHLIGHT
CLICK 2.00 moles, times grams per mole, gives us 34.1 grams.
CLICK PAUSE END RECORDING
17.03 g NH3
2.00 mol NH3 x
= 34.1 g NH3
1 mol NH3
To convert moles to grams: Mass = moles x (Molar Mass)
= 2.00 mol x (17.03 g/mol) = 34.1 g

2. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08