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

Question

finding quite tricky help me plsssss

Answer 1

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

Answer 2

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. 

Answer 3

Use molaer mass and Avagodros number:

 

250 g O₂ x 1 mole o₂  x 6.022x10²³ molecules O₂

                 32 g o₂           1 mole O₂