# Chemistry: Percentage Composition by Mass (1 Viewer)

#### hs17

##### New Member
Question 6 A)

Carbon monoxide is 43% carbon by mass; nitrous oxide is 30% nitrogen by mass. For both gases, the remainder is oxygen. In a mixture containing 5g of each gas, what is the mass of oxygen present?

Question 7)

In a carbon monoxide and nitrous oxide mixture, an analyst determines that the mixture is 10% nitrogen by mass. What is the percentage of carbon by mass? Use the mass compositions given in part A of Question 6. (Hint: consider a 100g sample to work out the percentages(

#### TheShy

##### Member
6. So we have 5g of CO. We know that 43% of it is Carbon. By doing the calculations of 43% x 5, we get = 2.15g. And it also says the remaining is oxygen gas. Thus we can simly do 5-2.15 to get the amount of Oxygen. However, another way to do this question is to simply find 57% of 5g, since 43% of it is carbon and the rest is oxygen, so 100-43. You do the same for the nitrous gas aswell.

#### hs17

##### New Member
Thanks! Did you manage to do q7 by any chance? #### TheShy

##### Member
Thanks! Did you manage to do q7 by any chance?
Yeah nah, I coulnd't seem to find the answer to that. Might have to ask your teacher or jazz, sorry!

#### jazz519

##### Moderator
Moderator
Question 6 A)

Carbon monoxide is 43% carbon by mass; nitrous oxide is 30% nitrogen by mass. For both gases, the remainder is oxygen. In a mixture containing 5g of each gas, what is the mass of oxygen present?

There seems to be a mistake in the question that was given to you (or you made a typo). Nitrous oxide also known as dinitrogen oxide has the formula N2O, this doesn't have 30% nitrogen by mass it's actually 64%. The compound that would fit that description of 30% nitrogen is nitrogen dioxide.

So I've answered the question with nitrogen dioxide (NO2) instead (but anyways it will give same answers)

Carbon monoxide (CO): %(carbon) = 43%. %(oxygen)=100-43=57%
So then just multiply 5g by 57% i.e. m(oxygen) = 5x0.57 = 2.85g

Nitrogen dioxide (NO2): %(nitrogen) = 30%. %(oxygen)=100-30=70%
m(oxygen)=5x0.7 = 3.5 g

Question 7)

In a carbon monoxide and nitrous oxide mixture, an analyst determines that the mixture is 10% nitrogen by mass. What is the percentage of carbon by mass? Use the mass compositions given in part A of Question 6. (Hint: consider a 100g sample to work out the percentages)

If nitrogen = 10%, if we assume a 100g sample that means m(nitrogen) = 10g

The nitrogen can only come from the nitrogen dioxide.

This means if you have 10g of nitrogen this represents 30% of the mass of the nitrogen dioxide (question 6). So we can increase this number to give 100% and therefore the mass of the nitrogen dioxide

If 10g is 30% of NO2, that means 10/30 is 1% of NO2. So 10/30 x 100 is 100% of NO2. So the mass of NO2 in the mixture is 33.33... g

If NO2 is 33.33..g of the 100g sample, then m(CO) = 100-33.33...=66.66...g

We know from before that %(Carbon in CO) = 43%

m(Carbon) = 66.66... x 0.43 = 28.67g

Since it was a 100g sample a mass of 28.67g carbon means 28.67% carbon percentage by mass

• Drdusk and Pedro123