Determing Chemical Formulas from Mass %'s

[skyisblue]

Is there any quick way to determine the chemical formula from mass % compostion?

I would take the mass% and from there figure out the moles, then divide by smallest mole, then round-off or multiply to get the molecular formula...it takes a while actually to do this...is there a shortcut??

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[Lonely Sol]

Is there any quick way to determine the chemical formula from mass % compostion?

I would take the mass% and from there figure out the moles, then divide by smallest mole, then round-off or multiply to get the molecular formula...it takes a while actually to do this...is there a shortcut??

If you could post a problem up, it would be easier to see and maybe explain!

 

[skyisblue]

If you could post a problem up, it would be easier to see and maybe explain!

Ok Ok, here's a problem:

Compound contains 75.7% Carbon, 8.8% Hydrogen, and 15.55% Oxygen.

MM is less than 210g/mol

What's the chemical formula of this compound?

Is there any shortcut to solving this problem?

It's 1am Western time. I'm going to sleep. I won't see your response until tomorrow. Thanks Lonely!

 

[jdmsamurai]

i dunno about you but the first thing that comes into my head =

.757 * 210 / 12 = 13.5C's

.088 * 210 / 1 = 18.48H's

.1555 * 210 / 16 = 2O's

so something like

C13H18O2 plus/minus more H's?

 

[BodybldgDoc]

i dunno about you but the first thing that comes into my head =

.757 * 210 / 12 = 13.5C's

.088 * 210 / 1 = 18.48H's

.1555 * 210 / 16 = 2O's

so something like

C13H18O2 plus/minus more H's?

yup this is the easiest method to find the molecular formula of a compound.

 

[Lonely Sol]

Ok Ok, here's a problem:

Compound contains 75.7% Carbon, 8.8% Hydrogen, and 15.55% Oxygen.

MM is less than 210g/mol

What's the chemical formula of this compound?

Is there any shortcut to solving this problem?

It's 1am Western time. I'm going to sleep. I won't see your response until tomorrow. Thanks Lonely!

Okay well, first you look at how close the choices are, if they are far apart then you dont even did to write all the info down. FIst let say Carbon, they say its 75.7 grams, so in your mind try dividing that by 12, you will easily see that its ~6.3. Same thing with the others, so you will get:
C ~ 6.3____6.3/1___6.3
H ~ 8.8 ___8.81/1___8.8______(Just write these 3 #'s down on your paper)
O ~ 1____1/1_____1

Then, the empirical will be ~ C6H9O, MW of this is 97. Then you take 210/97 = 2

You MF then equals C12H18O2 (approx.)

SO basically write the ratio down on paper and divide the MW of empirical formula and divide by 210, Then times the number with the empirical to get MF.

Plz correct me if I am wrong!

 

[skyisblue]

Okay well, first you look at how close the choices are, if they are far apart then you dont even did to write all the info down. FIst let say Carbon, they say its 75.7 grams, so in your mind try dividing that by 12, you will easily see that its ~6.3. Same thing with the others, so you will get:
C ~ 6.3____6.3/1___6.3
H ~ 8.8 ___8.81/1___8.8______(Just write these 3 #'s down on your paper)
O ~ 1____1/1_____1

Then, the empirical will be ~ C6H9O, MW of this is 97. Then you take 210/97 = 2

You MF then equals C12H18O2 (approx.)

SO basically write the ratio down on paper and divide the MW of empirical formula and divide by 210, Then times the number with the empirical to get MF.

Plz correct me if I am wrong!

This is how I would solve it too! But what did you mean by looking at the choices first??

 

[skyisblue]

i dunno about you but the first thing that comes into my head =

.757 * 210 / 12 = 13.5C's

.088 * 210 / 1 = 18.48H's

.1555 * 210 / 16 = 2O's

so something like

C13H18O2 plus/minus more H's?

WOW, this is a really good way of doing these problems. Much much faster. Take a look at this Lonely. Might save us some time on the DATs.

This is the correct answer BTW.

 

[Lonely Sol]

WOW, this is a really good way of doing these problems. Much much faster. Take a look at this Lonely. Might save us some time on the DATs.

This is the correct answer BTW.

Yea, that looks easier then what I was doing!! I need to prepare for Gchem because I am having a little hard time with it. Thats where I miss the most points