Mercury's mass is 0.055 times that of Earth or 3.302×10^23 kg (2.263x10^22 slugs*) *The slug is an english unit of mass. It should be noted that the pound is a unit of weight. That is, it is a measure of the amount of force acting on a mass. The conversion value used to convert kilograms to pounds is only accurate when the mass is being acted on by a force that equal the pull of gravity created by Earth. Since we are discussing mercury here. That conversion does not apply. So, I gave the mass in force independent units of slugs.

Mercury's density is slightly less than the Earth's (see Density). That is, a portion of Mercury would weigh slightly less than an equal portion of the Earth. Mercury is smaller than the Earth and therefore has much less mass (see Mass). Mercury's smaller mass makes its force of gravity only about a third as strong as that of the Earth. An object that weighs 100 pounds on the Earth would weigh only about 38 pounds on Mercury. http://www.nasa.gov/worldbook/mercury_worldbook.html Mercury's mass is about 3.3 x 1023 kg. This is about 1/20th of the mass of the Earth. http://www.enchantedlearning.com/subjects/astronomy/planets/mercury/ The mass of a planet is the amount of matter it contains. Since objects attract each other gravitationally, and that gravitational force depends on mass, we can find a planet’s mass by measuring its effect on the motions of other objects passing nearby, such as comets, asteroids, or spacecraft. In this way, it has been found that Mercury is rather “light” compared to other planets, with a mass of 3.3 x 1023 kg. This means that about 18 Mercury's would equal the mass of Earth. http://btc.montana.edu/messenger/elusive_planet/fastfact_6.htm