## Percent Mass Composition

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Jacob Villar 2C
Posts: 105
Joined: Sat Aug 17, 2019 12:18 am

### Percent Mass Composition

Apologies if this question may seem rather simplistic, but in a given problem, why would a given percent mass value, i.e. 49.8% C, also be used in the process of calculating moles? Will the value always be based off of 100 grams?

Hiba Alnajjar_2C
Posts: 108
Joined: Fri Aug 09, 2019 12:17 am

### Re: Percent Mass Composition

No question is too simplistic! But to answer, one can assume that they have 100 grams of the compound during the process of looking for the empirical formula in order to simplify calculations (since all the percentages add up to 100). The ratio between elements would be the same if you had 100 grams or if you had 3500 grams, since it's a ratio. Assuming you have 100 grams just simplifies calculations. So, from there, you'd convert from grams to moles, divide by the smallest number of moles, and find the resulting ratio between elements. Hope this helps!

Ashley Tran 2I
Posts: 108
Joined: Thu Jul 11, 2019 12:17 am

### Re: Percent Mass Composition

When we are not given the exact mass of the elements in a compound, we can assume there are 100 grams of the compound and multiply that by the percent composition of each element to find the proportional mass of each element. Then, we would convert to moles and divide by the lowest value of moles to find the whole number ratios between the element, which is then the empirical formula. Note that we don't always have to use 100 grams. If the prompt already tells us the total mass of the compound and the percent composition of the elements in the compound, we can just multiply the percent by total mass to find the actual mass of each element and proceed to convert to moles as mentioned before.

Brian_Ho_2B
Posts: 221
Joined: Fri Aug 09, 2019 12:16 am

### Re: Percent Mass Composition

Jacobian wrote:Apologies if this question may seem rather simplistic, but in a given problem, why would a given percent mass value, i.e. 49.8% C, also be used in the process of calculating moles? Will the value always be based off of 100 grams?

Like others have mentioned, the 100g sample is just to simplify calculations when converting from mass percentages to mass in grams. If you wanted to, you can use any number of grams for the sample, even the mass of the given sample in the question. In the end, the moles of each element will have the same ratio to each of the other atoms in the compound. Just remember: after converting masses to moles, divide the subscripts by the lowest number and then multiply the subscripts by an integer to make it so that each subscript is a whole number. Essentially the 100g sample step is optional and mainly to make calculations easier.

RoshniVarmaDis1K
Posts: 51
Joined: Sat Aug 24, 2019 12:16 am

### Re: Percent Mass Composition

We are usually given the percent mass of an element when we need to figure out the empirical formula of a compound. It is important to note here that the exact mass of the compound or element here then does not matter; all that matters is the ratio of elements to each other.

We assume 100. grams simply because it makes calculations easy; you could assume any quantity you want.

We then convert grams to moles to figure out the mole ratios.

Angela Patel 2J
Posts: 110
Joined: Sat Aug 24, 2019 12:17 am

### Re: Percent Mass Composition

Why do we need to find the percent mass composition? What does it help us to find?

Jainam Shah 4I
Posts: 130
Joined: Fri Aug 30, 2019 12:16 am

### Re: Percent Mass Composition

APatel_4A wrote:Why do we need to find the percent mass composition? What does it help us to find?

Percent composition tells us how much a certain compound or molecule is comprised of one component or other. Knowing the ratio of elements within a substance allows to find the empirical formula. The empirical formula represents the ratio of atoms within the substance which we can only obtain if we use percent mass composition to find the mole ratios between each element.

Kishan Shah 2G
Posts: 132
Joined: Thu Jul 11, 2019 12:15 am

### Re: Percent Mass Composition

Adding on, if the total mass of the compound is given and the respective masses of each element are given like the example regarding Vitamin C in class, then you need to divide each respective mass by the total mass given and then find the percent mass composition. From this point on you can continue by the original method of converting the percentage to grams, assuming its a 100g sample, for simplicity.

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