[jamesmith134]

Hello,

I have a problem with finding how many atoms of certain element are in a molecule.

Example:

How many atoms of Hydrogen are in 14.2 grams of (NH2)2CO; (answer in moles)
What I began doing is to find this molecule molecular mass which is 60.0556.

But I dont know what to do from here

Thanks

[Corribus]

What are the units of the molecular mass you determined?

[jamesmith134]

What are the units of the molecular mass you determined?

I don't know myself' I just did the sum of the number in the periodic table of the elements..
Can you maybe tell me what types of units are there and what are most common?> and show me with them?

THanks

[Corribus]

Ok, the units of molecular mass that you determined are grams of the substance per mole of the substance, usually abbreviated as grams per mole. So, one mole of (NH₂)₂CO weighs approximately 60.06 grams. If you have 14.2 grams of (NH₂)₂CO, how do you determine the number of moles that are present?

And from that, you need to determine how many hydrogens are present.

[jamesmith134]

Thanks, so if there are 60 grams per mole, so in 14 grams there are about 0.233 moles. but How do I take from that the moles of the hydrogen?

[Corribus]

Ok, I will lead you down the right path.

How many molecules of (NH₂)₂CO are in one mole of (NH₂)₂CO?
How many hydrogen atoms are in each molecule of (NH₂)₂CO?

[jamesmith134]

There are 6.022×10^23 molecules in 1 mole?
in each atom there are 4 atoms of Hydrogen?

*Update : So what I tried is: Find how many moles are in 14.2 grams of the molecules, which I got by ratio : 0.236 moles of the molecule in 14.2 grams.

Now I need to find the % of the Hydrogen from the total molecule which is 4/8=0.5? and multiply 0.5 by 0.236? or I miss a part

[jamesmith134]

[sorry delete ]
[due to double post of previous]

[Corribus]

You keep changing what exactly you are supposed to determine. Is the the number of hydrogen atoms? Is it the percent of all atoms that are hydrogen atoms? Is it the number of moles of hydrogen atoms?

What is the exact question being asked?

[jamesmith134]

It says I need to find the number of atoms of Hydrogen in moles (It's confusing..) Can you help me ?

I'm stuck at the stage where I get that there are 0.236 moles in 14.2 grams of (NH2)2CO. How do I get from here to how many atoms of Hydrogen (in moles) are there?

[Corribus]

Ok, I interpret that to mean they want the number of moles of hydrogen atoms.

So, if you know there are 0.236 moles of (NH2)2CO and you know there are 6.02 x 1023 molecules of (NH2)2CO in each mole of (NH2)2CO, and you know how many atoms of hydrogen are in each molecule of (NH2)2CO, all you need left is how many atoms of hydrogen are in each mole of hydrogen.

Here's the train of thought:

[tex]\frac {\text{0.236 moles (NH2)2CO}}{} \text{ X } \frac{6.02\text{x}10^{23}\text{ molecules (NH2)2CO}}{\text{1 mole (NH2)2CO}} \text{ X } \frac{\text {4 atoms of hydrogen}}{\text{1 molecule (NH2)2CO}} \text{ X } \frac{\text{1 mole of hydrogen atoms}}{6.02\text{x}10^{23}\text{ atoms of hydrogen}}[/tex]

Usually we take a conceptual shortcut and just realize that there are 4 moles of hydrogen atoms 1 every one mole of (NH2)2CO molecules, but I wanted to write it all out so you can see where the logic lies.

This is the kind of unit conversion you need to become proficient at in order to be successful in general chemistry.

Now you should have everything - just multiply the #s to get the correct answer. Note that from here you have access to all kind of information: how many grams of hydrogen are in 14.2 grams of (NH2)2CO, what the percentage by mass of hydrogen is in the molecule, and so forth. But it all starts with unit conversions and understanding stoichiometry.

[jamesmith134]

Thank you man, everything is understood now 
I just prefer to separate it to steps rather than 1 long equation because this way I understand what I do better.