The below lesson is from http://molarmath.info/
Ratio # 1: The Number of Particles in a Mole
602,200,000,000,000,000,000,000, or 6.022 x 10^23 = # of particles in a mole.
In chemistry, particles usually means molecules, ions or electrons.
The value is given BY DEFINITION based on experiments in the real world.
This relationship may be written in one of two ways:
Remember, it doesn’t matter WHAT the particles are. The number of particles in a mole is ALWAYS the same.
Now let’s use this ratio in solving two example problems. In the first few examples, I go into excruciating detail, but after you get a feel for this kind of problem solving, it goes very quickly and is really very EASY. (My assumption is that you already understand scientific notation and significant figures.)
Problem 1: Find the Number of Particles from Moles
You have 3.0 moles of substance. How many particles of substance do you have?
Analysis: You are given moles and are asked to find particles.
So you start with units of moles and end up with units of particles in your answer.
Before you even begin to worry about the numbers, be sure to place the starting and ending units where they need to go.
Notice that we placed the units where they go first, without even worrying about the numbers.
We also left a blank space to write in the conversion ratio, once we determine what that is.
Next, we need to choose the correct version of the ratio between moles and particles to make the conversion work.
Concentrate on just finding the UNITS which will cancel out moles and leave us with particles.
Below we see the two possible ways that units in the Conversion Ratio may be placed. Which one do you think is correct?
If you chose possibility “A,” you are correct.
That placement allows us to cancel the unit “moles” and keep the unit “particles.”
Now that our units are placed correctly, we can insert the numbers.
We start with 3.0 moles of substance.
# of particles in a mole is constant, so our conversion factor is constant.
(I have also placed grey numeral “1’s” into the problem to help us remember that even when there are no numbers or units written, the number “1” is always there.)
Our final steps are to check the unit cancellation, do the calculation, determine significant figures and put the answer in final form.
Problem 2: Find the Number of Moles from Particles
Instead of being given moles as in Problem 1, you are now given a number of particles and must convert back to the units of moles.
You have 2.56 x 10^25 particles of substance. Find out how many moles you have.
Analysis: You are given particles and are asked to find how many moles you have.
So this time you start with particles and end up with units of moles in your answer.
Follow the same pattern as before. Begin by placing your units first.
Next, choose which version of the conversion ratio you need :
to cancel out the units you DON’T want and end up with the units you DO want.
In this case, if you chose possibility “B,” you are correct.
Particles will cancel and you will end up with moles, which is what we want.
Now put the numbers in, cancel units, do the calculations, determine significant figures and report the answer in final form.
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# of particles in a mole = 6.022 x 10^23. This is ALWAYS true.
In doing conversions, we can use the ratio either “right side up” or “upside down,” depending on what we’re converting FROM and what we’re converting TO.
The steps to solving these kinds of problems are:
1. Place the starting and ending units in their correct locations.
2. Choose the correct conversion ratio to make unwanted units cancel.
3. Plug in the numbers and do the calculation.
4. Put the answer in final form. EASY!
Ratio # 2: The Volume of a Mole of Gas at STP
This quantity is used specifically ONLY for particles in the gaseous state.
Its value is FIXED at 22.4 Liters per mole at STP (Standard Temperature and Pressure).
STP was arbitrarily set and agreed upon by the scientific community to mean 25 degrees C and 1 atmosphere pressure, the pressure at sea level.
Note: At DIFFERENT temperatures and pressures, the volume occupied by one mole of gas is different and can be calculated using the laws of relationship known as the “gas laws.”
Nevertheless, for many calculations in chemistry, volume is measured and compared at STP, so this is a useful ratio.
The ratio representing the volume of a mole of gas at STP may be written in either of two ways:
Problem 3: Find the Number of Liters from Moles
You have 4.0 moles of pure nitrogen gas at STP. How many Liters of gas do you have?
Analysis: You are given moles and are asked to find Liters.
You start with units of moles and end up with units of Liters. Place your units as shown below.
Choose the correct conversion ratio to make the unwanted units cancel.
If you chose “A,” you are correct. Next, insert the numbers, cancel units, do the calculation, round to the correct number of significant figures and report the answer in final form.
Problem 4: Find the Number of Moles from Liters
You have 49.7 Liters of oxygen gas at STP. How many moles do you have?
Analysis: You are given Liters and are asked to find moles.
You start with units of Liters and end up with units of moles.
You should place your beginning and ending units as shown below.
From here, which conversion factor do you choose?
The one which has Liters on top or moles on top?
Choose the correct conversion factor, plug in the numbers and proceed through to the final answer.