chemistry 1

Stoichiometry TutorialFinding Molar Mass

Moles and Molar MassThe mole is the "counting unit" used by chemists to indicate the number of atoms, ions, molecules, or formula units present in a particular chemical sample. The mole is similar to other counting units that you've used before....pair (2), dozen (12), and gross (144). One mole of a compound contains Avogadro's number (6.022 x 1023) of molecules (molecular compound) or formula units (ionic compound). The molar mass of a compound tells you the mass of 1 mole of that substance. In other words, it tells you the number of grams per mole of a compound. The units for molar mass are, therefore, grams/mole.To find the molar mass of a compound:1. Use the chemical formula to determine the number of each type of atom present in the compound.2. Multiply the atomic weight (from the periodic table) of each element by the number of atoms of that element present in the compound.3. Add it all together and put units of grams/mole after the number.

Example:Find the molar mass of sodium carbonate, Na2CO3.Na2x23.0=46.0

C1x12.0=12.0

O3x16.0=48.0

molar mass=106.0 g/mole

For many (but not all) problems, you can simply round the atomic weights and the molar mass to the nearest 0.1 g/mole. HOWEVER, make sure that you use at least as many significant figures in your molar mass as the measurement with the fewest significant figures. In other words, never let your molar mass be the measured value that determines how many signficant figures to use in your answer!

Example: What is the molar mass of calcium nitrate, Ca(NO3)2?Ca1x40.1=40.1

N2x14.0=28.0

O6x16.0=96.0

molar mass=164.1 g/mol

Notice that you must be very careful when you're counting the number of atoms present in this compound. The subscript "2" after the parentheses indicates that there are 2 nitrate ions (NO3-). In order to figure out how many N atoms are present, you must multiply the subscripts (in this case 1 x 2 = 2). In order to figure out how many O atoms are present, you must multiply the subscripts (in this case 3 x 2 = 6). Notice that the subscript outside the ( ) affects only the atoms inside the ( ) and not the Ca ion.

Practice ProblemsCalculate the molar mass for each of the following compounds:1. Fe2O32. AgNO33. Pb(Cr2O7)24. Ca(ClO4)2Stoichiometry TutorialFinding Molar Mass- Answers

1. The molar mass of Fe2O3is 159.7 g/mol.Fe2x55.85=111.70

O3x16.0=+ 48.0


2. The molar mass of AgNO3is 169.9 g/mol.Ag1x107.9=107.9

N1x14.0=14.0

O3x16.0=+ 48.0


3. The molar mass of Pb(Cr2O7)2is 639.2 g/mol.Pb1x207.2=207.2

Cr4x52.0=208.0

O14x16.0=+ 224.0


4. The molar mass of Ca(ClO4)2is 239.1 g/mol.Ca1x40.1=40.1

Cl2x35.5=71.0

O8x16.0=+ 128.0


Molar Mass as a Conversion FactorChemical reactions occur on a molecular (or atomic) basis. When we want to plan a chemical reaction, we must know the number of molecules (or atoms or formula units) of a compound that should be present in the flask. The easiest way to keep track of (or count) the number of molecules (etc) present is to use the mole. Although we always plan experiments using the number of moles of reactants and/or products, we commonly carry out those experiments by measuring out a specific mass of each reactant. Once we plan the experiment, we then need a way to convert between grams and moles of a particular substance.As you will recall, the molar mass of a compound tells you the mass present in 1 mole of that substance. In other words, it tells you the number of grams per mole of a compound. Since the molar mass relates two different sets of units, grams and moles, it can be used as a conversion factor. The easiest way to convert between grams and moles (or vice versa) is by using dimensional analysis....i.e. keeping track of your units as you go along.Converting Grams A to Moles A1. Write down the units that you are looking for (including the formula for the compound) and an equal sign:mol NaCl=

2. 3. Write down the number of grams you were given in the problem:mol NaCl=12.5 g NaCl

4. 5. Put a multiplication sign after the mass and draw a linemol NaCl=12.5 g NaClx________

6. 7. Write the molar mass (including the units) above and below the line you've drawn in such a way that the grams cancel out. put the number of grams in the molar mass on the bottom put "1 mol" on the topmol NaCl=12.5 g NaClx1 mol NaCl

58.5 g NaCl

8. Cancel out the "grams" and do the math.mol NaCl=12.5g NaClx1 mol NaCl=0.214 mol NaCl

58.5g NaCl

9.

Converting Moles A to Grams A1. Write down the units that you are looking for (including the formula for the compound) and an equal sign:g NaCl=

2. Write down the number of moles you were given in the problem:g NaCl=0.122 mol NaCl

3. 4. Put a multiplication sign after the moles and draw a lineg NaCl=0.122 mol NaClx________

5. Write the molar mass (including the units) above and below the line you've drawn in such a way that 6. the moles cancel out. put the number of grams in the molar mass on the top put "1 mol" on the bottomg NaCl=0.122 molNaClx58.5 g NaCl

1 mol NaCl

7. Cancel out the "mol" and do the math.g NaCl=0.122molNaClx58.5 gNaCl=7.14 g NaCl

1mol NaCl

8.

Practice Problems:1. 12.5 g CaCl2= _______ mol CaCl22. 10.0 g Na2SO4= _______ mol Na2SO43. 16 g NaOH = _______ mol NaOH4. 125 mg NaOH = ______ mol NaOH5. 0.125 mol CaCl2= _____ g CaCl26. 0.25 mol Na2SO4= ______ g Na2SO47. 1.55 mol NaOH = ______ g NaOH

1. 12.5 g CaCl2=0.113 mol CaCl2molCaCl2=12.5gCaCl2x1 molCaCl2=0.113 mol

111.1gCaCl2CaCl2

2. 10.0 g Na2SO4=0.0704 mol Na2SO4mol Na2SO4=10.0g Na2SO4x1 mol Na2SO4=0.0704 mol

142.0g Na2SO4Na2SO4

3. 16 g NaOH =0.40 mol NaOHmol NaOH=16g NaOHx1 mol NaOH=0.40 mol NaOH

40.0g NaOH

4. 125 mg NaOH =3.13 x 10-3mol NaOHmol NaOH=125mg NaOHx1gx1 mol=3.13 x 10-3mol

1000mg40.0gNaOH

The only "twist" on this problem is that you first must convert mg to grams. The problem is then identical to the previous problem in terms of the approach to solving it. Notice that the answer was given in scientific notation. Your calculator should have read 0.003125. Remember that the answer should be reported to 3 signficant figures so it was rounded to 0.00313 and then converted to scientific notation because it is a small number.5. 0.125 mol CaCl2=13.9 g CaCl2gCaCl2=0.125molCaCl2x111.1 gCaCl2=13.9 gCaCl2

1molCaCl2

6. 0.25 mol Na2SO4=36 g Na2SO4gNa2SO4=0.25molNa2SO4x142.0 gNa2SO4=36 g

1molNa2SO4Na2SO4

The answer before rounding was 35.5 g. However, the answer must be reported to 2 significant figures. Therefore, it was rounded to 36 g.7. 1.55 mol NaOH =62.0 g NaOHg NaOH=1.55mol NaOHx40.0 g NaOH=62.0 g NaOH

1mol NaOH

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