1. How many moles of CH4 are in 4.00 g of CH4?
QUESTION:
How many moles of CH4 are in 4.00 g of CH4?
A mol, B mol, C mol
Molar mass of CH4: g/mol
moles CH4
1 mol CH4 =
mass CH4
16.04 g NH3
How many moles of CH4 are in 4.00 g of CH4?
A mol, B mol, C mol
PAUSE
By now, you should know how to calculate the molar mass given a formula.
CLICK For CH4, you should be able to calculate that the molar mass is grams per mole.
To do a mass to mole 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 moles of methane
CLICK
to the mass
CLICK CLICK
is equal to
the ratio of 1 mole
to grams
We can rearrange this equation to solve for the moles of methane. Essentially, what we would end up doing is
multiply the given mass of methane, 4.00 grams
CLICK by the ratio... 1 mole methane over grams
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, 4.00
CALLOUT “3 significant figures”
has only three significant figures grams has four significant figures,
CALLOUT and the number 1 in the numerator is exact.
CALLOUT
4.00 divided by gives us , which we round off to
Therefore, the correct answer is choice A moles.
Here’s another way of looking at this calculation.
CLICK if we want to convert grams to moles..
we simply divide the mass
HIGHILIGHT
by the molar mass
HIGHLIGHT
CLICK 4.00 grams divided by grams per mole, equals moles
CLICK PAUSE END RECORDING
1 mol CH4
4.00 g CH4 x
= mol CH4
16.04 g CH4
To convert grams to moles: Moles = Mass / (Molar Mass)
= 4.00 g / (16.04 g/mol) = mol

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