When 85.0 g of ch4 are mixed with 160. g of o2 what is the maximum amount of co2 that can be produced? ch4 + 2o2 ? co2 + 2h2o?

In order to determine the maximum amount of carbon dioxide that may be formed, we must determine the limiting reactant.
The limiting reactant is the reactant is the one that is present in an amount less than that which is required stoichometrically. The molar ratio of reactants is:

CH₄ : O₂ = 1 : 2

Now, we compute the moles of both carbon dioxide and oxygen keeping in mind that:
moles = mass / Mr

CH₄:
moles = 85 / 16 = 5.31

O₂:
moles = 160 / 32 = 5

The molar ratio present is:
1 : 1

Thus, we know that oxygen is the limiting reactant. We will calculate the moles of carbon dioxide formed from the moles of oxygen.

Stoichomterically
O₂ : CO₂ = 2 : 1 
Moles of CO₂ that will be formed:
5/2 = 2.5

Mass of CO₂ = 2.5 * 44
Mass of CO₂ = 110 grams

110 grams of carbon dioxide will be formed

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.