Mass and linear absorption coefficients of air for Cr K(alpha) X-rays

[asdf1]

for this question:
calculate the mass and linear absorption coefficients of air for Cr K(alpha) radiation. Assume that air contains 80 percent nitrogen and 20 percent oxygen by weight and has a density of 1.29*10^(-3) g/(cm^3)

my problem: i have no idea where to start.

 

[inha]

Mass absorption coeffs. for mixtures are calculated as
$$\frac \mu \rho = \Sigma_i w_i \frac {\mu_i} {\rho_i}$$ where the index i runs over the components, w_i are the weighing factors for the components and mu and rho are the abs. coeff and density for the element i.
The linear abs. coeff. is straight forward to get from that.

 

[asdf1]

but the question doesn't give any information about the rho's, so how do you know the values?

 

[inha]

Densities are standard tabulated material. Look the up somewhere.

 

[asdf1]

ok, thank you very much!