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Grahams Law Molecules at the same temperature have the same average kinetic energy. (T KE) KE is proportional to the speed of the molecules and the mass.

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Presentation on theme: "Grahams Law Molecules at the same temperature have the same average kinetic energy. (T KE) KE is proportional to the speed of the molecules and the mass."— Presentation transcript:

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2 Grahams Law

3 Molecules at the same temperature have the same average kinetic energy. (T KE) KE is proportional to the speed of the molecules and the mass of the molecules. KE = ½ mv 2

4 Grahams Law Diffusion – a gas spreading out Grahams Law deals with the effusion of two gases into each other. Effusion – a gas moving through a small hole

5 Grahams Law Gas A Gas B Consider two gases at the same temperature and pressure in a box with two partitions, separated by a wall with a hole that can be opened.

6 Grahams Law Gas A Gas B Gas A effuses into Gas B, and … Gas B effuses into Gas A, so that … …eventually both gases will be evenly distributed in the box.

7 Grahams Law Gas A Gas B The ratio of the rates at which Gas A and Gas B effuse into each other is … … inversely proportional to the square roots of their molar masses.

8 Grahams Law Gas A Gas B The bottom line: the lighter the gas, the faster it moves!

9 Grahams Law Gas A Gas B Suppose nitrogen and an unknown gas, both at the same temperature and pressure, are in the box. The rates of effusion are …

10 Grahams Law Gas A Gas B … moles/L/min for the nitrogen and mol/L/min for the unknown. What is the molecular weight of the unknown?

11 Grahams Law Gas A Gas B Start with the equation for Grahams Law

12 Grahams Law Gas A Gas B Square both sides, then solve for the molecular mass of compound x

13 Grahams Law Gas A Gas B

14 Grahams Law Gas A Gas B M x = 17.0g/mol

15 Grahams Law Gas A Gas B What is the unknown gas? The gas is smelly, dissolves in water to make a basic solution, and has a molar mass of 17 g/mol.

16 Grahams Law Gas A Gas B Ammonia gas, NH 3, has a strong smell, and...

17 OH - Grahams Law Gas A Gas B … it reacts with water … … it reacts with water … NH 3 (g) + HOH(l) NH 3 (g) + HOH(l) … which is a basic solution, OH - NH 4 + +

18 Grahams Law Gas A Gas B 1 x x 1.0 = 17.0 g/mol … and ammonia, NH 3, has a molar mass of 17.0 g/mol.


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