Somthing for the brighter ones..

ellis182 · Junior Member Joined: 16 Mar 2005 Posts: 4

Guys just a really easy question i need clarification with..

How many moles of O-atoms in 25.7g of CaSO4?

I know moles = mass/molar mass

so..

25.7/(40.8 + 32.07 + 4x16) = .188

Is this the answer or do i multiply .188 by 4 because there is 4 oxygen atoms in CaSO4?

Also..

How many molecules in 13.72g oh hydrazine? N2H4? i dont even know where to start! Embarassed

If anyone could help me at all it would be much appreciated!

RobJim · Posted: Wed Mar 16, 2005 10:36 am Post subject:

Here's how you do this:

First, you calculate how many moles of CaSO4 there are. It looks like you did that properly, and unless your numbers or your math is wrong, there are 0.188 molecules of CaSO4.

In each molecule of CaSO4, there are four oxygen atoms. So, yes, you multiply the 0.188 by 4 to figure out how many moles of oxygen atoms are in your sample.

ellis182 · Junior Member Joined: 16 Mar 2005 Posts: 4

Thanks so much for answering my first question!

However, could anyone help me with the second one?

How many molecules in 13.72g of hydrazine N2H4?

RobJim · Posted: Wed Mar 16, 2005 11:49 am Post subject:

First, find how many moles are in the sample. Then, use a conversion factor that converts moles to molecules.

Is that enough help, or would you like more details?