Question

You have two beakers, one filled to the 100-mL mark with sugar (which has a mass of 180.0g) and the other filled to the 100-mL mark with water (mass of 100.0g). You pour all the sugar and all the water together in a bigger beaker and stir until the sugar is completely dissolved.
Which of the following is true about the mass of the solution? Justify your answer.
a)it is much greater than 280.0g
b)it is somewhat greater than 280.0g
c)it is exactly 280.0g
d)it is somewhat less than 280.0g
e)it is much less than 280.0g

Answer 1

i thought you just add the two masses together but since the sugar is dissolved i wasnt sure....

Answer 2

i think it is it is much greater than 280.0g @fynn76

Answer 3

Well, let's do a little thought experiment. Suppose after you've mixed up the sugar and the water, you put the mess on the stove and heat it up. The water starts to boil, of course. You carefully collect every bit of the steam and pipe it to a cool bottle, where it condenses back to the pure water.

When you're all done, you have a bottle of pure water again, plus a pan full of the recrystallized sugar. What will each weigh?

Answer 4

it would weigh the same as before because of law of conservation of mass...right? but how could something weigh EXACTLY what you started with? is this just a hypothetical situation assuming that nothing is lost?

Answer 5

Yes, it's a hypothetical, but what do YOU mean by "exactly," hmm? If you can precisely define "exactly," then I will precisely define how exactly the masses would be the same.

What I think you'll find yourself doing is saying that the more effort you put into doing the weighing very carefully -- very delicate scales, transferring the materials exquisitely carefully -- the more precise your answers would be, and IF it turns out that the mass is conserved a little bit better every single time you improve how careful you are -- then you are willing to believe that if you were INFINITELY careful you would, indeed, find that the mass is EXACTLY the same. It is a limiting process, in other words.

But let's return to the thought experiment. So you started off with sugar and water with together weighed some amount, then you mixed them together, and then separated them again -- and you believe the mass of the separate components after you separate them will be the same as when you started. What does that tell you about the in-between step, when they are mixed? Is there a way they could weigh less -- but only for a little while?

Answer 6

Let's try another thought experiment. Suppose you take your sugar and your water, and you divide each of them up into little chunks. A tiny cube of sugar plus a tiny beaker of water. Millions of each. Leaving aside the weight of the containers (let's assume you correct for that), do you think the weight of all those millions of bits of sugar and water, added up together, will be different from the weight of the sugar and water when they were separate?