## anonymous one year ago What is the mass of He in a uniform cylindrical tube, 5mm radius, 0.1m length. Pressure at z=0 is 0.06atm, pressure at z=0.1 is 0.02atm . Temperature of the system is 298K. There is another gas in the tube that causes the pressure gradient but I don't believe that information is necessary. I know to use the idea gas law, however, I can't figure out how to account for the pressure difference when computing mass of the He in the system.

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1. IrishBoy123

first do we need to assume the shape of the gradient or is that a given? i will assume linear here so we can say $$p(z) = 0.06 - 0.4 \ z$$ the for the equation itself, written so that mass is in there, with M = Molar mass, R,T = constant $p \ V = m\frac{R \ T}{M_m}$ we consider a small element of the cylinder within which we can apply the ideal gaw law, because the pressure gradient in small element $$\to 0$$ |dw:1442050459814:dw| we can say $p(z) \ dV = dm\frac{R \ T}{M_m}$ $dV = \pi R^2 dz$ $\pi \ R^2 \ (0.06 - 0.4 \ z) \ dz = dm\frac{R \ T}{M_m}$ $\pi \ R^2 \int\limits_{0}^{0.1} \ (0.06 - 0.4 \ z) \ dz = \frac{R \ T}{M_m}\int\limits_{0}^{m_0} dm$ $m_o = 0.004 \ \frac{M_m}{R \ T} \pi \ R^2 = 0.04 \ V \frac{M_m}{R \ T}$ IOW you can use the average of the 2 pressures if the gradient is linear.