Molarity Calculations



Sec 7.3

Molarity Calculations and Stoichiometry

…same as before but starting with something different

You have seen calculations that start with moles, mass, volume at STP and molecules. Now there is one more thing to start with and it is the last. MOLARITY. You have seen it before.

Molarity = moles or M = mol

Liters L

This is the equation that you need to start with BEFORE you can get to moles. So you must rearrange it to solve for moles

mol = M x L that’s it.

So, if you are given a volume and a molarity and asked to find moles, mass, volume at STP or molecules you must first rearrange this formula to get moles. Then do the same procedure to get the ratio then the final answer.

This new first step can be done separately and not in the same line as the other equations

1. How many grams of copper will react to completely replace silver from 208.0 mL of 0.100M solution of silver nitrate, AgNO3 ?

Cu + 2 AgNO3 → Cu(NO3)2 + 2Ag

M = mol/L

mol = M x L

0.100M x 0.2080L = 0.02080moles AgNO3

0.0208 mol AgNO3 x _1 mol Cu_ x _63.5 g Cu_ = 0.660 g Cu

2 mol AgNO3 1 mol Cu

2. When the copper replaces the silver in 208.0 mL of 0.100M silver nitrate, how many grams of silver will be produced?

M = mol/L

mol = M x L

0.100M x 0.2080L = 0.02080moles AgNO3

0.0208 mol AgNO3 x _2 mol Ag _ x _107.9 g Ag_ = 2.25 g Ag

2 mol AgNO3 1 mol Ag

3. How many grams of Mg will react to completely replace hydrogen from

14.0 mL of 0.800M solution of HCl?

Mg + 2HCl → MgCl2 + H2

M = mol/L

mol = M x L

0.800M x 0.0140L = 0.0112 moles HCl

0.0112 mol HCl x _1 mol Mg_ x _24.3 g Mg_ = 0.13608g Mg = 0.0136 g

2 mol HCl 1 mol Mg

4. How many litres of H2 (g) at STP are formed from the complete reaction of 1.31g of magnesium with 100.0 mL of 1.552 M hydrochloric acid?

Mg + 2HCl → MgCl2 + H2(g)

M = mol/L

mol = M x L

1.552M x 0.1000L = 0.1552 moles HCl

0.1552 moles HCl x _1 mol H2_ x _22.4L = 1.173824 L H2 = 1.173L

2 mol HCl 1 mol

Sec 7.3

Molarity Calculations and Stoichiometry

…same as before but starting with something different

You have seen calculations that start with moles, mass, volume at STP and molecules. Now there is one more thing to start with and it is the last. MOLARITY. You have seen it before.

Molarity = moles or M = mol

Liters L

This is the equation that you need to start with BEFORE you can get to moles. So you must rearrange it to solve for moles

mol = M x L that’s it.

So, if you are given a volume and a molarity and asked to find moles, mass, volume at STP or molecules you must first rearrange this formula to get moles. Then do the same procedure to get the ratio then the final answer.

This new first step can be done separately and not in the same line as the other equations

1. How many grams of copper will react to completely replace silver from 208.0 mL of 0.100M solution of silver nitrate, AgNO3 ?

Cu + 2 AgNO3 → Cu(NO3)2 + 2Ag

2. When the copper replaces the silver in 208.0 mL of 0.100M silver nitrate, how many grams of silver will be produced?

3. How many grams of Mg will react to completely replace hydrogen from

14.0 mL of 0.800M solution of HCl?

Mg + 2HCl → MgCl2 + H2

4. How many litres of H2 (g) at STP are formed from the complete reaction of 1.31g of magnesium with 100.0 mL of 1.552 M hydrochloric acid?

Mg + 2HCl → MgCl2 + H2(g)

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