Molar mass for compounds 
The formulas for compounds are familiar to you. You know the formula for hydrogen peroxide is H_{2}O_{2}. It should be reasonable that the weight of a formula unit can be calculated by adding up the weights for the atoms in the formula.The weights are read from the periodic table. The formula weight for H_{2}O_{2} = weight from hydrogen + weight from oxygen 
The formula weight for H_{2}O_{2} = 2 H atoms x 1.008 amu + 2 O atom x 16.00 amu = 34.016 amu 
The molar mass for H_{2}O_{2} = 34.016 grams H_{2}O_{2} 
How many grams of hydrogen are needed to give 3. moles of hydrogen? 
1. Calculate the molar mass for hydrogen. Look up the atomic weight/mass in the periodic tableThe molar mass for hydrogen is 1 mole H_{2} = 2.016 grams H_{2} 
2. determine the mass needed to provide 3. moles of hydrogen. 1 mole H_{2} = 2.016 grams H_{2}; 2 mole H_{2} = 4.032 grams H_{2} 3 mole H_{2} = 6.048 grams H_{2} The practical way is to multiply the molar mass by the number of moles. This converts mole to grams (3 mole H_{2} )(2.016 grams H_{2}/ 1 mole H_{2} ) = 6.048 grams H_{2 } Here the label moles H_{2} will cancel out. return to top of page 
Example 
How many moles of water are in a liter of water? Assume 1 liter = 1 kilogram water 
1. Calculate the formula weight(mass) for water, H_{2}O.

2. Add up the masses from all the atoms in the formula The formula weight for water is 1.008 amu + 1.008 amu + 16.00 amu = 18.016 amu 
3. Determine the molar mass for water. Molar mass is a mass in grams that is numerically the same as the formula weight. 1 mole H_{2}O = 18.016 grams H_{2}O = 18. grams H_{2}O ( to the nearest gram ) 
4. Convert 1000 grams of water to moles. The "conversion factor" is the molar mass. (1000 grams H_{2}O )(1 mole H_{2}O/ 18. grams H_{2}O) = 55.55 moles H_{2}O This is 56 moles H_{2}O (rounded to 2 sf) _{ return to top of page} 
How many moles of sulfur dioxide, SO_{2} (g), are in 2000 grams of the gas? 

1. Look up the atomic weights in the periodic table for S and O.

2.Calculate the formula weight for SO_{2} . Add up the masses from all the atoms in the formula The formula weight for sulfur dioxide is 32.07 amu S + 2 x (16.00 amu O) = 64.07 amu SO_{2} 
3. Determine the molar mass. Molar mass is a mass in grams that is numerically the same as the formula weight. 1 mole SO_{2} = 64.07 grams SO_{2 }= 64. grams SO_{2} (to the nearest gram ) 
4. Convert 2000 grams of SO_{2} to moles. The "conversion factor" is the molar mass. (2000 grams SO_{2} )(1 mole SO_{2 }/64. grams SO_{2 }) = 31.25 moles SO_{2} = 31 moles SO_{2 }(rounded to 2 sf) 
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