Elemental calcium reacts with water at room temperature to form calcium hydroxide and hydrogen gas as shown in this equation. sodium undergoes a similar reaction with h2o. what would be the coefficient for sodium in a similar reaction?

Reaction of Elemental Calcium with Water is as follow,

                        Ca  +  2 H₂O    →    Ca(OH)₂  +  H₂↑

When Sodium Metal is reacted with Water it produces NaOH and eliminates Hydrogen gas as shown in following equation,

                         2 Na  +  2 H₂O    →    2 NaOH  +  H₂↑

In both cases corresponding Hydroxides along with Hydrogen gas is produced.

The answer should be c.

Mass is the amount of matter in an object.  weight is the measurement of the pull of gravity on an object.  say for example, you travel to the moon which has a different gravitational field compared to the gravitational field of earth. (gravitational fields: earth = 1, moon = 0.165)  now when you are on the moon, the pull of gravity on you is different than on the earth. and as weight is the measurement of the pull of gravity on an object, that means yes, your weight can and will change on the moon.  however, on the moon, on earth or even on jupiter, the amount of mass inside of you remains the same. meaning your physical mass remains constant.  in conclusion it is possible for your weight to change as your mass remains constant.