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calculate the number of molecules in 250g of oxygen....

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3 Answers

You can use the molar mass of oxygen gas (O2) and Avogadro's number as conversion factors in this problem.  The molar mass is good for "g→mol" or "mol→g" conversions.  Avogadro's number is good for "molecules→moles" or "moles→molecules" conversions.
 
Avogadro's number is the number of things (6.022 x 1023) in a mole of things (e.g., there are 6.022 x 1023 molecules of O2 in one mole of O2).
 
Molar masses are found on the periodic table.  The molar mass of an element is the mass, in grams, of a mole of atoms for each element.  It is 16.00 g/mol for O.  Because there are two O atoms in O2, the molar mass for O2 is 32.00 g/mol.
 
The calculation can be planned out like this, because we have conversion factors for each of the two steps.
 
grams O2 → mol O2 → molecules O2
 
And the calculation steps are set up this way:
 
250 g O2 x (1 mol O2 / 32.00 g O2) x (6.022 x 1023 molecules O2 / 1 mol O2
 
   = 4.7 x 1024 molecules
A couple of facts that you need for this type of chemistry
 
1. there are 6.022 x 1023 molecules in 1 mole of anything
2. Every mole of Oxygen weighs 31.9988 grams 
 
Solution
 
(250g) (1 mole/31.9988g) (6.022 x 1023 molecules/mole) = 4.7 x 1024 molecules of Oxygen
 
Notice that the answer has the right number of significant figures. The trick is to start with what you are given, and cancel the units out so that you are left with the desired unit (which is usually stated in the question itself. In this case, it is molecules of Oxygen) on the top of the last fraction. 

Comments

Use molaer mass and Avagodros number:
 
250 g O2 x 1 mole o2  x 6.022x1023 molecules O2
                 32 g o2           1 mole O2 

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