The H-R diagram is useful in studying all of these about a star
A. except for its brightness b. location in the sky c. temperature
D. magnitude

b.location in the sky (If the question is saying:The H-R diagram is useful in

studying all of these about a star except for)

Explanation:

The theoretical HR diagram plots the temperature of the star vs their luminosity (brightness) and the observational HR diagram plots color of the stars vs the absolute magnitude. Depending on mass, stars will also go through specific evolutionary stages that can be seen through the location of the star on the HR diagram. However, the HR diagram will not have data on the location of the star in the sky.

the ideal gas law equation is as follows

pv = nrt

where p - pressure

v - volume

n - number of moles

r - universal gas constant

t - temperature

so if the pressure, volume and temperature are already known

we are left with n and r

since r is the universal gas constant that has a known fixed value then r too is known

so we are left with 'n'

once we know temperature volume and pressure

we can find the number of moles of gas present using the ideal gas law equation

this is not my own answer, this is an answer for the same question by the user blahblahmali. i do not claim ownership of this account.