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Question Video: Converting Kilojoules to Kilocalories Chemistry • 10th Grade

A food manufacturer determines that the energy content of one slice of their new bread is 400 kJ. What value is this in kilocalories? Give your answer to 1 decimal place.

03:18

Video Transcript

A food manufacturer determines that the energy content of one slice of their new bread is 400 kilojoules. What value is this in kilocalories? Give your answer to one decimal place.

Energy is a word you may have heard used in a variety of contexts. But in a strict scientific sense, energy is the capacity for doing work. An important concept is that it cannot be created nor destroyed; it can only be transferred to other forms.

Let’s look at how we measure energy. The SI unit for energy is a joule, which is defined as the amount of thermal energy needed to raise the temperature of one gram of water by one divided by 4.184 degrees Celsius. Now that we have discussed energy and joules, let us begin to answer the question in order to convert 400 kilojoules to kilocalories.

Quite often, the numbers we are dealing with are very large, so we can use the kilojoule. One kilojoule equals 1000 joules. We can use dimensional analysis and set up conversion factors. Remember, when you set up the conversion factor to convert kilojoules to joules, put the like unit on opposite sides of the fraction so that they will cancel out.

Another common unit used to measure energy is the calorie. Calories are often used in nutrition and can be found on food labels. The calorie is the quantity of energy needed to raise the temperature of one gram of water by one degree Celsius. It is equivalent to 4.184 joules exactly. One calorie is equivalent to exactly 4.184 joules.

Now we can convert joules to calories using the following conversion factor. Remember, when you set up the conversion factor to convert joules to calories, put the like units on opposite sides of the fraction so that they cancel out. We could also convert calories to kilocalories using a similar conversion factor as the one used to convert joules to kilojoules. Remember one kilocalorie equals 1000 calories.

If we follow the equation on the top, we will take 400, multiply it by 1000, divide by 4.184, and then divide by 1000. But you may notice that there is a little redundancy there, and we can actually convert directly from kilojoules to kilocalories, just as you can directly convert from joules to calories. Doing this calculation gives us our answer of 95.60229 in units of kilocalories. However, we need to give our final answer to one decimal place. Rounding to one decimal place gives us our final answer of 95.6 kilocalories.

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