A solution is prepared in a 50 mL volumetric flask by mixing 3 mL of 0.00312 M NaSCN with 15mL of 0.20M Fe(NO3)3. Enough 0.25 M HNO3 is added to fill the flask to the 50 mL mark. Before equilibrium was established, what was the initial concentration of SCN- in the solution?

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.

it is the most commonly accepted one