ALL mole chemistry problems you have from here on out are simply extensions of the skill of being able to find the correct conversion ratio which cancels unwanted units and leaves wanted units. EASY!
Using Two Ratios in One Problem
Problem 5: Find the Number of Molecules from Liters.
In this problem, you will be using TWO conversion factors to get the answer.
How many molecules of hydrogen gas are in 54.6 Liters of pure hydrogen at STP?
Analysis: You are asked to start with Liters and end with molecules.
(The clue that you end with molecules is given from the words “how many.”)
While we have no direct one-shot conversion between Liters and molecules, we DO have “Liters to moles” and “moles to molecules.”
So this time we can use TWO conversion factors to get our answer.
Set it up like this:
Next, find the conversion factors needed to do the conversion.
Finally, plug in the numbers and do the calculation.
Notice that the entire strategy for solving problems like this, even with more than one conversion factor, is simply setting up the starting and ending units and then filling in the gaps with ratios either “right side up” or “upside down” to make unwanted units cancel and keep the units you want in the answer. EASY!
Molar mass is different from the first two ratios, because it is NOT a single constant value which never changes. Although any given substance has the same unchanging, constant molar mass, each different substance has its own molar mass. Look at the three examples below.
A mole of hydrogen molecules (which are made up of two hydrogen atoms bonded together) has a mass of 2 grams.
A mole of helium gas has a mass of 4.0 grams.
A mole of water has a mass of 18.0 grams.
If you do not know how to find the molar mass of a substance, click here. [Not active yet.]
Now let’s use molar mass to solve problems.
Problem 6: Find Moles from Grams.
How many moles of water are in 45.0 grams of water?
Analysis: The words “how many” point to the needed “ending units.” The words “are in” indicate the “starting units.”
For water, the two possible ratios are:
Now choose which of the above conversion ratios can be used to solve this problem.
Hopefully you chose the conversion factor shown below.
Finally, write in the numbers, do the calculation, determine significant figures and report the answer in final form.
Problem 7: Find Grams from Moles.
How many grams of diatomic hydrogen gas are in 5.7 moles of hydrogen?
Analysis: The words “how many” point to the ending units. The words “are in” point to the starting units.
Start by setting up the units
Choose the correct conversion ratio.
Write in the numbers, cancel units, do the calculation, determine significant figures and report the answer in final form.
Using Two Ratios in One Problem
Now let’s use molar mass ratios along with the first two molar ratios to solve problems.
Problem 8: Find Liters at STP from Grams.
How many Liters of space will 25.3 grams of methane gas occupy at STP?
Analysis: The words “how many” indicate the ending units. What are the starting units? And what are the conditions? (STP) The condition of STP is important, because the conversion ratio of gas volume at STP is valid ONLY at standard temperature and pressure. That’s why is must be stated explicitly in the problem.
Set up the problem.
Find and choose the correct conversion ratios.
Put in the numbers, cancel units, do the calculation, determine correct number of significant figures and report the final answer.
If you like the way this material is being presented, consider buying HOLY MOL-EE! Chemistry, which is on SPECIAL right now.