[fatghost13]

A mixture of N2 and O2 gases has a density of 1.3 gm/L at standard temperature and pressure (STP).  Find the mole fraction of each gas.   
Please help to solve the problem .thx

[sjb]

A mixture of N₂and O₂ gases has a density of 1.3 gm/L at standard temperature and pressure (STP).  Find the mole fraction of each gas.   
Please help to solve the problem .thx

Do you know any formulas that may help?

[fatghost13]

 I have no idea about that , can you give a hand to me ?

[sjb]

I have no idea about that , can you give a hand to me ?

What is the definition of density?

[fatghost13]

I have no idea about that , can you give a hand to me ?

What is the definition of density?

density = mass / vol
1.3gm/L = m / 1L
m = 1.3 g

is it right?

[sjb]

Do you know any other formulas that relate e.g. volume to amount of material for gases?

[StibAMDG]

You can use the density of gases formula, d = PM/RT, where P is the pressure, M is the molar mass (in this case a weighted average of the two gases' molar masses), R is the ideal gas constant, and T is the temperature.  Once you have solved for the averaged molar mass, it is possible to find the mole fractions by solving the simultaneous equations
X(N2) + X(O2) = 1
and
28.0 g/mol * X(N2) + 32.0 g/mol * X(O2) = M.

[fatghost13]

You can use the density of gases formula, d = PM/RT, where P is the pressure, M is the molar mass (in this case a weighted average of the two gases' molar masses), R is the ideal gas constant, and T is the temperature.  Once you have solved for the averaged molar mass, it is possible to find the mole fractions by solving the simultaneous equations
X(N2) + X(O2) = 1
and
28.0 g/mol * X(N2) + 32.0 g/mol * X(O2) = M.

Noted with many thanks