How many molecules are in 10.0 g of O_2 gas at STP?

2 Answers
Feb 27, 2016

How many molecules at STP? The same number of molecules in 10.0 g of LIQUID OXYGEN at LOW TEMPERATURES.

Explanation:

You specified a mass of 10.0 g. Clearly, this is a finite number of dioxygen molecules. This same finite number (which is N_Axx(10.0*g)/(32.0*g*mol^-1), N_A = "Avogadro's number") is the same mass whatever the state, solid, liquid, or gas.

Of course, at STP, dioxygen is a gas, but 10.0 g is still 10.0 g. We could calculate its volume at STP, which is 22.4 L xx its molar quantity, approx. 8*L.

Feb 27, 2016

There are 1.51xx10^23"molecules O"_2" in "10.0 g O"_2".

Explanation:

STP is irrelevant - it's a detractor. Mass is not dependent on temperature or pressure. The number of molecules of "O"_2" are dependent only on the moles of "O"_2" in "10.0 g O"_2".

Determine the molar mass of "O"_2", which is (2xx15.998"g/mol")="39.998 g/mol".

1"mol of molecules"=6.022xx10^23"molecules"

You will make the following conversions:

color(red)("mass O"_2)rarrcolor(green)("mol O"_2)rarrcolor(blue)("molecules O"_2)**
by 1) dividing the given mass "O"_2" by its molar mass, and 2) multiplying mol "O"_2" by 6.022xx10^23"molecules".

color(red)10.0cancel(color(red)("g O"_2))xx(color(green)(1)cancelcolor(green)("mol O"_2))/(color(green)(39.998)cancelcolor(green)("g O"_2))xx(color(blue)(6.022xx10^23"molecules O"_2))/(cancelcolor(blue)("mol O"_2))=color(blue)(1.51xx10^23"molecules O"_2") rounded to three significant figures