# Molar Ratios and Mass Relationships in Chemical Equations

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```					               Molar Ratios and Mass Relationships in Chemical Equations
This handout demonstrates how to solve a typical stoichiometry problem involving mass relationships
and molar ratios.

Problem #1: Given this reaction answer the following questions.
CaCl2 (aq) + Na2C2O4 (aq)  CaC2O4 (s) + NaCl (aq)
a) If 0.043g of oxygen was produced, how many grams of chlorine reacted?
b) How many moles of CaCl2 reacted?
c) How many moles of NaCl were produced if 4.39g of Na2C2O4 reacted?
STEP 1: Make sure the equation is balanced!
CaCl2 (aq) + Na2C2O4 (aq)  CaC2O4         ( s)   + NaCl   (aq)

Ca             1                                                    1            Ca
Cl             2                                                    1                Cl
Na             2                                                    1            Na
C2O4           1                                                    1          C2O4
The equation is not balanced. Adding a 2 in front of NaCl in the products yields:
CaCl2 (aq) + Na2C2O4 (aq)  CaC2O4 (s) + 2 NaCl (aq)
Ca             1                                                    1            Ca
Cl             2                                              2 1                    Cl
Na             2                                              2 1                Na
C2O4           1                                                    1          C2O4
Now the equation has equal numbers of each atom in both reactants and products.
CaCl2 (aq) + Na2C2O4 (aq)  CaC2O4 (s) + 2 NaCl (aq)
a) If 0.043g of oxygen was produced, how many grams of chlorine reacted?
STEP 2:Set up the units going from what we have to what we want.
what we have                                             what we want
0.043 g O =      ? g Cl

gO       mole O     mole CaC2O4     mole CaCl2              mole Cl          g Cl

gO          mole O    mole CaC2O4         mole CaCl2         mole Cl

Cancel the units until the desired unit is the only one left.
STEP 3:Calculate any molar masses (formula weights) needed and fill in the numbers using the
balanced equation to find the molar ratios.
0.043 g O 1 mole O         1 mole CaC2O4 1 mole CaCl2              2 mole Cl      35.45 g Cl
= 0.048 g Cl
16.00 g O       4 mole O    1 mole CaC2O4 1 mole CaCl2 1 mole Cl
molar mass    molar ratio        molar ratio         molar ratio    molar mass

b) How many moles of CaCl2 reacted?
STEP 2:Set up the units going from what we have to what we want.
what we have                                                     what we want
0.043 g O = ? mole CaCl 2

gO         mole O      mole CaC2O4            mole CaCl2

gO              mole O      mole CaC2O4

Cancel the units until the desired unit is the only one left.
STEP 3:Calculate any molar masses (formula weights) needed and fill in the numbers using the
balanced equation to find the molar ratios.
0.043 g O 1 mole O               1 mole CaC2O4 1 mole CaCl2
= 6.7 x 10-4 mole Cl
16.00 g O            4 mole O        1 mole CaC2O4
molar mass         molar ratio         molar ratio

c) How many moles of NaCl were produced if 4.39g of Na2C2O4 reacted?
STEP 2:Set up the units going from what we have to what we want.
what we have                                                     what we want
4.39 g Na2C2O4 = ? mole NaCl

g Na 2C2O4 mole Na 2C2O4              mole NaCl
g Na 2C2O4      mole Na 2C2O4

Cancel the units until the desired unit is the only one left.
STEP 3:Calculate any molar masses (formula weights) needed and fill in the numbers using the
balanced equation to find the molar ratios.
4.39 g Na2C2O4 1 mole Na 2C2O4                 2 mole NaCl
= 0.0655 mole CaCl2
134.0 g Na2C2O4 1 mole Na 2C2O4
molar mass              molar ratio

As you can see, the same basic steps work for all three problems. Remember your significant figures!

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