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### Topic: Problem with computing calculation  (Read 4936 times)

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#### lilstens

• Guest ##### Problem with computing calculation
« on: September 01, 2004, 04:00:42 PM »
Ok, so here is the problem...

We have 25g of water. How much hydrogen is needed to make this in grams?

I am way confused right now...I know that there is a 2:1 ratio between hydrogen and oxygen but I would like some help setting up the equation. #### movies

• Organic Minion
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• Gender: • Better living through chemistry! ##### Re:Problem with computing calculation
« Reply #1 on: September 01, 2004, 06:20:21 PM »
The ratio that you speak of is the "molar ratio" of hydrogen atoms to oxygen atoms.  That's where you want to start.  First figure out the number of moles of water that you have using the molecular weight of water.  Then use the ratio of moles of hydrogen atoms to moles of water, or better yet, the ratio of moles of molecular hydrogen (H2) to moles of water.  Then figure the mass of hydrogen from that number of moles.

A short-cut would be to use the weight percent of hydrogen in water, but I'm guessing that's not the idea in this problem.  The answer is the same though.

#### lilstens

• Guest ##### Re:Problem with computing calculation
« Reply #2 on: September 01, 2004, 06:33:39 PM »
ok, can you show me how to find the molecular weight of water? Also, please show me the shortcut method. Thx #### movies

• Organic Minion
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• Gender: • Better living through chemistry! ##### Re:Problem with computing calculation
« Reply #3 on: September 01, 2004, 06:39:49 PM »
All you need to get the molecular weight is a periodic table.  Take the number of each atom present in the molecule and multiply it by the atomic mass of the atom, which is listed on the periodic table, then add them all up.  For example, methane, CH4: 12.011 + 4*1.0079 = 16.0426.

The short cut would be to figure out the percentage of the mass that is hydrogen and then multiply that percentage by the total mass that you have.  You would be doing essentially the same thing with the molar conversion, but it's more stepwise.

#### ssssss

• Guest ##### Re:Problem with computing calculation
« Reply #4 on: September 02, 2004, 06:08:05 AM »
As movies said,you should 1st know how to calculate the molecular mass of a compound.Its mentined and its quite easy.

Secondly you should know whenever we are considering Chemical equations we are dealing with Moles not the weight.

2H2 + O2-->2H2O

Now as you can see there is 2:1 ratio of Hydrogen and oxygen.This is the Molar ratio and dont confuse it with weight.You will convert these moles in weight[in gms] seperately.

Now 2 moles of Hydrogen gives 2 Moles of water.And now converting it into weight we have.4gms of Hydrogen gave 36 gms of water[Molecular weight of H2 =1+1 and molecular weight of H2O=1+1+16].Now if you want to calculate how much of hydrogen is required for 25 gms of water,you can do it quite easily its just like a simple unitary method.