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Topic: Percentage and formulae  (Read 3172 times)

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Offline willpfc

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Percentage and formulae
« on: February 07, 2009, 11:56:22 AM »
Hi, Pls cud u help me solve this question and tell me how to work it out.Thx

22.5% of the mass of a sample of phosphorus chloride is phosphorus.
What is the formua of phosphorous chloride? (Ar values: P=31, Cl=35.5)

Offline ARGOS++

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Re: Percentage and formulae
« Reply #1 on: February 07, 2009, 12:06:38 PM »

Offline mahar.sean

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Re: Percentage and formulae
« Reply #2 on: February 07, 2009, 12:11:38 PM »
     l am going to venture a guess, an relatively uneducated one, that you should pay attention to the word "mass".  Mass is not weight, which creates problems with using molar values.  You may need to change the mass to weight using: w=mg. Weight equals mass times acceleration due to gravity, which if I recall correctly can be stated as 9.8 meters per second squared.  Unfortunately I have forgotten the units for mass.
     Please excuse me if I am erroneous, I have the intent to help.

Offline Arkcon

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Re: Percentage and formulae
« Reply #3 on: February 07, 2009, 12:22:28 PM »
It is a little sloppy, to be sure, but chemists do use the term molecular mass and molecular weight, as synonymous.  This is not to say, that an item's weight and mass are equivalent. 

FWIW, the SI units for mass is actually, grams.  Since weight is defined as mass times force of gravity, we use, IIRC, newtons.  Recently, I mentioned in another thread, for those stuck in the Imperial measurement system, lbs. is the units for weight, the units for mass, is the slug.  Which I always found really funny. 

But all this has no bearing at all, on the original problem.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline mahar.sean

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Re: Percentage and formulae
« Reply #4 on: February 07, 2009, 12:27:55 PM »
Argos;
     I forgot that the problem can be solved independent of units when percentages are involved.  Thank you for helping willpfc, where I may have just confused the matter.

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