4) How many grams of F2, gas occupy 4.9 L at STP?

We know two equations that can help as

n = m/Mr

n = V/Vm

So, combining the two equations above:

m/Mr = V/Vm

m = V×Mr/Vm

m = 8.31 gr

*To calculate the Mr of "F2" we need to multiply by 2 the atomic mass of F, which we can find onlu by looking it up in a table (probaly in the back of a chemistry book, or online). We see that it is equal to 19, so Mr will be equal to 38.

**The equations are derived easily. Lets take for example the first one. 1 mole of the substance contains Mr gr of it. So, how many gr do "n" moles contain?

8.31g

Explanation:

First, we calculate the molar mass of F2.

MM of F2 = 2 x 19 = 38g/mol

But,

1mol of a gas occupy 22.4L at stp.

This implies that 1mole(i.e 38g) of F2 occupies 22.4L.

Therefore, Xg of F2 will occupy 4.9L i.e

Xg of F2 = (38 x 4.9) / 22.4

Xg of F2 = 8.31g

Therefore 8.31g of F2 occupies 4.9L

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I hope this helps  :)