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Food Calorimetry: Common Mistakes

Food Calorimetry: Common Mistakes
okay i want to talk about two of the most

common

mistakes

that people make when they're doing

food

calorimetry

problems okay here's what i mean by

food

calorimetry

let's say that we want to know how many calories are like in a potato chip okay we'll take this potato chip and we'll light it on fire underneath a container of water oftentimes if you're doing this in lab the container water that you use is like a soda can okay so the burning potato chip is going to release
food calorimetry common mistakes
heat and that is going to heat up the water in this container so then after it burns for a while the temperature of the water rises so to figure out how many calories of heat energy or in this potato chip we measure the difference in temperature from beginning to end and then we plug that number as well as others into this equation q equals m times c times delta t so this is what i mean by

food

calorimetry

now i want to go through a practice problem which might be useful even if you're not
making any of these

mistakes

but i want to go through this practice problem and show you the points where people make

common

mistakes

as i work through this problem i'm going to intentionally make a mistake see if you can catch it some students plan to measure the amount of calories in a potato chip they fill a metal can this container here with 60 milliliters of water at 15 degrees celsius they take a 1.9 gram potato chip and set it on fire under the can that's what's going on right
here after the chip has stopped burning the temperature of the water has risen up to 85 degrees celsius so the question asks how many calories are in the chip as you probably know we're going to use this equation the first thing let's solve for is delta t okay so delta t is final temperature minus initial temperature so the final temperature here is 85 degrees celsius 85 degrees celsius minus minus the initial temperature which is 15 degrees celsius that's what we started at okay so
if we subtract this we're going to find that the delta t equals 85 minus 15 which is 70 degrees celsius okay so now we've got delta t let's plug some numbers into this equation so we can say that q equals m we want mass that's going to be 1.9 grams times c which is the specific heat which is one calorie divided by grams times degrees celsius this is how many calories it takes to take one gram of water and raise its temperature up by one degree celsius so one calorie over grams
food calorimetry common mistakes
times degrees celsius times delta t 70 degrees celsius equals multiply these three numbers together and i get 130 calories and i'll also point out that my units cancel i have grams up here grams down there degrees celsius down here degrees celsius up there okay does that make sense it's totally wrong this is probably the most

common

mistake that people make when they're doing these kind of questions do you know what's wrong about this take a look at this equation here is
what's wrong with it it's this number right here 1.9 grams 1.9 grams is the mass of the potato chip but i don't want to be using the master potato chip i want to be using the mass of the water here's why let's look back what's actually going on here okay so we've got this potato chip we set it on fire that causes it to release its heat energy okay so it gives all of its heat energy up and then the container of water above the potato chip that absorbs all of the energy
the potato chip is releasing so if we want to figure out how much energy the potato chip released all we got to do is calculate how much heat energy the water absorbed so that's what we're looking for the amount of heat energy absorbed by the water that's what we're using this equation for okay but let's look more specifically at the variables q is the heat absorbed by water which will tell us how much heat the potato chip gave off okay q heat absorbed by water so it's
all about the water we're focusing on the water here m has to be the mass of the water because we're focusing on the water okay c sometimes written as s is the specific heat of the water how much energy does it take to raise the temperature of this water and finally the delta t is a change of temperature of the water so this is all about the water it's all about the amount of heat absorbed by the water so we have to use the mass of the water that's in this container we can't
food calorimetry common mistakes
use the mass of the potato chip because this is all about the water you must use the mass of the water okay i think this can be tricky because of the way the problem was written okay here's why i'm betting that you look at this equation and you think ah-ha m i'm going to need a mass mass masses is in grams oh check it out there's a mass right here 1.9 grams i better put that in the equation okay notice that the problem doesn't give you right away a mass for water okay so it
only gives you 60 milliliters of water that's not a mass it's a volume so that's why i think that sometimes people take a look at these problems they say oh 1.9 grams that's what i want to be using so this leads us to the second big mistake that people make i don't give you in this question a mass for water how can you figure it out here's how water is very special and it's good because it's easy for water and only for water one milliliter equals one gram so i
didn't give the mass of the water here but i gave milliliters and since you know that one milliliter equals one gram this is very easy to realize that 60 milliliters of water equals 60 grams of water okay so we can replace 1.9 grams the massive potato chip which was wrong with 60 grams which is the mass of the water okay keep in mind that it is only water that has one mil equals one gram it's not any liquid it's just water water's special it's super easy to remember this okay
so anyway 60 grams times the specific heat times delta t i can cancel my grams out it's going to give me a much bigger number it's going to give me 4 200 calories okay now keep in mind if you were doing these calculations to find the number of calories in the potato chip these are what we call little c calories they're not

food

calories also known as kilo calories or calories spelled with a big c so you need to divide this number by a thousand in order to get the number of

food

calories okay but this is 4200 little c calories anyway let's just review the two things that we talked about here the first thing we said is that when you're doing this q equals m times c times delta t equation you're solving for the amount of heat that's absorbed by the water that's given off by the burning potato chip or whatever it is a burning cheeto or a burning frito it doesn't matter okay you're in in any of those situations you're trying to calculate the
heat absorbed by the water which means you have to use the mass of the water not the mass of whatever's burning okay so that's the first thing massive water has to be your m the second thing is that if you are not given a mass of water if you're just given milliliters of water it's super easy to figure out what the mass is and that's because one milliliter of water is one gram of water so that's how you can always know what the mass of water is even if it's not given
in the problem so keep these two things in mind and you will avoid the two most

common

mistakes

that people make when they're doing these sort of problems good luck