# Example Problem with Complete Solution

1B-3 : Units and Carbon Dioxide Emissions 4 pts
A new, energy-efficient home refrigerator consumes about 350 kW-h of electricity per year. Assume the electricity is generated by burning fossil fuels. When fossil fuels like coal, oil, natural gas and gasoline are burned, most of
the carbon in the fuel burns completely to form carbon dioxide (CO2). CO2 is a greenhouse gas that contributes to global warming and that is undesirable.  In a typical natural gas power plant, 0.5 kg of CO2 is produced for each
kW-h of electricity generated. Consider a city with 500,000 homes with one refrigerator in each home. How much CO2, in metric tons, is produced by the refrigerators in the city in one year?

Keep careful track of units and it will not be difficult.
A kW-h is unit of energy.  A Watt is energy per time and when you multiply by hours (time) you are left with units of energy.  A kW-h is kW times hours. It is the amount of energy consumed in an hour by a device that uses 1 kW of power (1 kJ/s for 3600 sec).
Given: mCO2 0.5 kg CO2 / kW-h Power 350 kW-h/house/year
N 500000 houses
Find: MCO2 ??? mton CO2 / year
Assumptions: None.
Equations / Data / Solve:
The total mass of CO2 produced per year is the product of the rate of CO2 production per kW-h, the rate at which power is used by each house and the number of houses.
Eqn 1
Check the units in this equation :
Eqn 2
Plug in the values : MCO2 8.75E+07 kg CO2 / year
Unit conversion factor : 1 metric ton = 1000 kg
Therefore : MCO2 87500 mton CO2 / year
Answers: MCO2 87500 mton CO2 / year