What mass of oxygen gas is produced by the reaction of 5.2 g of water? Be sure to have the correct # of sig figs.

Question

Green plants use light from the Sun to drive photosynthesis. Photosynthesis is a chemical reaction in which water (H2O) and carbon dioxide (CO2) chemically react to form the simple sugar glucose (C6H12O6) and oxygen gas (O2). What mass of oxygen gas is produced by the reaction of 5.2 g of water? Be sure to have the correct # of sig figs.

Answer 1

First write and balance the equation for CO₂ + H₂O ---> C₆H₁₂O₆  + O₂

 

     6 CO₂    +  6 H₂O   ------->  C₆H₁₂O₆     +   6 O₂

 

 

5.2 g H₂O   x ( 1 mol H₂O / 18.015 g H₂O)  x (6 mol O₂/6 mol H₂O)  x  (32.00 g O₂/mol O₂)  =  9.2 g O₂  

 

(Since 5.2 g has only 2 sig fig's, there are only 2 sig fig's in the answer)

 

The easiest way to set up a problem like this is to use a box (grid) so you can clearly see the units cancel. The conversion factor from the balanced equation, namely 6 moles of oxygen are produced from 6 moles of water, can be simplified to 1 mol of oxygen per 1 mole of water if desired, otherwise, just cancel the 6's.

 

5.2 g H₂O   |  1 mol H₂O      |  6 mol O₂    |  32.00 g O₂     =  9.2 g O₂

                 |   18.015 g H₂O |  6 mol H₂O |   mol O₂