The Notes: How many molecules of water are formed when 8 g of oxygen reacts with NH3? 4NH3+5O2------>6H2O+4NO

Saturday, May 16, 2009

How many molecules of water are formed when 8 g of oxygen reacts with NH3? 4NH3+5O2------>6H2O+4NO

This is a stoichiometry problem. The balanced reaction is:

$\displaystyle~{4}{N}{H}_{{3}}+~{5}{O}_{{2}}-\gt~{6}{H}_{{2}}{O}+~{4}{N}{O}$

Step 1: Convert grams of oxygen ( $\displaystyle~{O}_{{2}}$ ) to moles.

First, determine the molar mass of $\displaystyle~{O}_{{2}}$ . The molar mass of a substance is determined by multiplying the atomic mass of each element in the substance by its subscript and then adding the resulting answers together.

Molar mass of $\displaystyle~{O}_{{2}}$ = 2(15.9994) = 31.9988 g/mol

Second, convert the given grams of oxygen to moles by dividing by the molar mass.

(8 g)(1 mol/ 31.9988 g) = 0.250 mol $\displaystyle~{O}_{{2}}$

Step 2: Multiply the answer from Step 1 by a ratio of the coefficients of $\displaystyle~{O}_{{2}}$ and $\displaystyle~{H}_{{2}}{O}$ .

First, identify the coefficient of each substance as indicated in the chemical equation.

The coefficient of $\displaystyle~{O}_{{2}}$ is 5.

The coefficient of $\displaystyle~{H}_{{2}}{O}$ is 6.

Second, write the coefficients as a ratio. The ratio can be written two ways:

5 moles $\displaystyle~{O}_{{2}}$ /6 moles $\displaystyle~{H}_{{2}}{O}$ OR 6 moles $\displaystyle~{H}_{{2}}{O}$ /5 moles $\displaystyle~{O}_{{2}}$

Choose the ratio with the moles of the given substance ( $\displaystyle~{O}_{{2}}$ ) on the bottom.

Third, multiply the answer from Step 1 with the coefficient ratio that you just chose.

(0.250)(6 mole/5 mole) = 0.30 mole $\displaystyle~{H}_{{2}}{O}$

Step 3: Convert moles of $\displaystyle~{H}_{{2}}{O}$ to molecules.

Multiply the answer to Step 2 with the conversion factor, 1 mole = 6.02 x $\displaystyle~{{10}}^{{23}}$ molecules. Orient the conversion factor such that unit "mole" is in the denominator. This causes the unit "mole" to cancel out, leaving your answer in "molecules."