please help me with this chemistry conversion

Question

Convert 92 kwh into calories.... My teacher got 7.9 x 10^4... It's right , but I have no idea how the exponent 4 is there. I just have no idea how my teacher got that . PLEASE HELP. I HAVE A QUIZ TOMORROW

Answer 1

First, let's start by turning 1 kwh into calories (your answer means you're actually using kilocalories, which are typically written with a capital C, so I'll use that notation).  I'm assuming your teacher taught you the conversion 4.2 kilojoules per Calorie.

 

So 1 kwh means 1 kilowatt * 1 hour.

 

1 watt is 1 joule/second, and there are 3,600 seconds per hour, so 1kwh = 1 kilojoule/second *3,600 seconds = 3.6*10^3 kilojoules.

 

So, 92 kwh = 92*3.6*10^3 kilojoules = 3.312*10^5 kilojoules.  If there are 4.2 Calories per kilojoule, 92 kwh = 3.312*10^5 kilojoules * (1 Calorie / 4.2 kilojoules) = 7.9*10^4 Calories.

 

Hope this helps!