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mass percentage problem and strange density problem?

Submitted by kripsamson on Tue, 08/18/2009 - 23:13

1) Hi i dont need the answer or anything i would just love if someone could explain to me where i must start with this problem and what exactly i should be thinking when i see MASS PERCENT. thanks!.

A 0.100 g sample of a carbon - hydrogen - oxygen compound is combusted in a stream of pure oxygen and produces 0.220 g CO2 and 0.239 g  . What is the mass percent of hydrogen in the sample?[/b]

2) this just got me nowhere, i dont know where i should start what i should start with or anything, totally confused :(.

What thickness (in cm) is the silicon block (density = 2.33 g/cm3) that is 2.87 cm wide and long necessary to react with 91.3 g of Cr2O3 by the reaction: 3 Si (s) + 2 Cr2O3 (s) ? 3 SiO2 (s) + 4 Cr (l)?

Mass percent means the mass proportion of the element in the compound if the compound weighed 100 units. eg the mass percent of Carbon in methane (CH4) = (12/16) x 100. This works out at 75% which means that in terms of mass, Carbon makes up 75% of the total mass of the compound.

(I have used 12 as relative atomic mass of C, and 1 for H, so that the molecular weight of CH4 = 12+(1x4) = 16g
You must probably use more accurate values for atomic mass

If relative atomic mass of C, H and O are 12, 1 and 16 respectively, then 12+(16x2)g or 44g of CO2 contains 12g of Carbon, and (1x2)+16 g or 18g of water contains 2g of hydrogen.

You have the masses of CO2 (0.220g) and H2O (0.239g I think)  made in the reaction so you can calculate the masses of C and H that were in the compound based on the information in bold above.

(The total mass of the compound is the mass of C, H and O. So by adding the masses of C and H together, you can find the mass of O in the compound - but I don't think you need this in this calculation).

Percent of Hydrogen in compound = (mass of H/mass of compound) x 100

In the second question, you need to calculate the mass of silicon needed to react with 91.3g of Cr2O3 based on the info in the equation that 3 moles of Si react with 2 moles of Cr2O3.

Start by finding the masses of 1 mole of each of the above eg if the relative atomic mass of Si = 28.09, then 1 mole weigh 28.09g and 3 moles weighs 3 x 28.09g
Do the same for Cr2O3 (approx 52x2+16x3)= 152g per mole or 304g for 2 moles. Please check and use the atomic masses expected for your course.

This tells me that about 84g Si reacts with 304g Cr2O3
So find the mass of Si that reacts with 91.3g Cr2O3.

You have the density of Si so rearrange this to find the volume of this mass of silicon

Volume = Mass/Density.

Now you have the volume. So find the thickness
LxBxThickness = Volume calculated.

Plug in the length (L) and Breadth (B), and the volume you calculated to find the thickness