Search 75,766 tutors
FIND TUTORS
Ask a question
0 0

Grams-moles

Tutors, please sign in to answer this question.

3 Answers

The periodic table gives molar masses for each element in units of grams/mole.  You can use that as a conversion factor to convert grams to moles, or moles to grams.  Examples:
 
  • Convert 7.25 moles of neon to grams.
7.25 moles of neon x (20.18 g Ne / 1 mol Ne)
= 146 grams of neon
 
  • Convert 7.25 grams of neon to moles.
7.25 grams Ne x (1 mol Ne / 20.18 g Ne)
= 0.359 moles of Ne
 
 
If you're dealing with molecules, you need to calculate the molar mass of the compound first, and then use that as your conversion factor.  Examples:
 
  • Convert 45.00 grams of CO2 to moles.
Molar mass of CO2 = (1 x 12.01) + (2 x 16.00)
= 44.01 g/mol
 
45.00 g CO2 x (1 mol CO2 / 44.01 g CO2)
= 1.022 moles CO2
 
  • Convert 45.00 moles of CO2 to grams.
Use molar mass from previous example.
 
45.00 moles CO2 x (44.01 g CO2 / 1 mol CO2)
= 1.980 x 103 g CO2
 
 
Try these:
 
1. What is the mass, in grams, of 2.4 x 108 moles of H2O?
 
2. How many moles of sucrose (C12H22O11) are in 75 grams of sucrose?
The Periodic Table lists the “Atomic Weight” of each element. The Atomic Weight is given in units of gram/mole, so that’s a key piece of information we need to perform this conversion.

To convert grams to moles:
 
If you’re converting a single element, such as silicon, read off the atomic weight from the Periodic Table and divide that into the number of grams of Silicon (Si) you’re converting:

Number of Moles of Si = Number of Grams of Si/Atomic Weight of Si
 
For 56.02 grams of Si:

Number of Moles of Si = 56.02 grams/28.09 grams per mole = 2 moles

If you’re converting a compound, you need to add up the atomic weight of the compound. So for a compound such as CO2, you’ll need to add the atomic weight of carbon and two atomic weights of oxygen since there are two oxygen atoms in CO2:

Atomic Weight of CO2 = (Atomic Weight of Carbon) + 2*(Atomic Weight of Oxygen)
Atomic Weight of CO2 = (12.01) + 2*(15.99) = 43.99 grams per mole

For 15 grams of CO2:

Number of Moles of CO2 = Number of Grams of CO2/Atomic Weight of CO2
Number of Moles of CO2 = 15 grams/43.99 grams per mole = 0.341 moles
 
To convert moles to grams, simply multiply the atomic weight of the element or compound by the number of moles you have:
 
For 2 moles of CO2:
 
Number of Grams = Number of Moles * Atomic Weight
Number of Grams = 2 * (43.99) =  87.98 grams of CO2
The simple explaination
 
1 mole= 6.022x1023 molecules, which also happen to way the same amount as any atomic mass of that element. So lets take 2 moles of Carbon (remember this is just an amount, like a "handful)
 
2 moles
 
well C=12 amu off the periodic table, so 1 mole of Carbon is 12 g (rounded) so
 
2 moles C •   12gC    
                1mole C
 
Multiply everything on top, divide by the bottom.
 
so what ever you start with, you put on the the bottom of the second step to can. out the units. In my example I canceled out moles and ended with g of Carbon. If you start with grams and want to get to moles, you have to have the grams on the bottom to cancel out g and get your desired (mol).
 
 
A mole is simply a universal ANMOUNT that we can use as a unit to compare amounts too. They just happened (kinda) to take 12 g's of Carbon-12 (isotope with 6p, 56n) and found out how many atoms there where.... 602200000000000000000000 (6.022x1023) molecules, or the things that will react, in that amount... This one-one ratio is why the weight of 6.022x1023 molecules is ALWAYS equal to it's atomic weight. If you have the same amount of bowling balls and tennis balls, the bowling  balls will weigh more, but there will still be the same amount. Same concept.
 
so I tried to keep it short up top, then if you wanted any further info I gave more detail.