Answer:
MIXTURE
Explanation:
A mixture is a substance composed of a combination of other different substances. These component(s) of a mixture are physically combined, meaning that there is no chemical linkage between the individual components/constituents of a mixture.
This is the case of the gravel described in this question. The components of gravel can be separated using physical means because they are not chemically bonded to one another, hence, no chemical reactions are needed to separate different parts of gravel into pure substances. This makes gravel a MIXTURE.
First, we have to remember the molarity formula:

Part 1:
In this case, our solute is sodium nitrate (NaNO3), and we have the mass dissolved in water, then we have to convert grams to moles. For that, we need the molecular weight:

Then, we calculate the moles present in the solution:

Now, we have the necessary data to calculate the molarity (with the solution volume of 200 mL):

The molarity of this solution equals 0.2339 M.
Part 2:
In this case, we have the same amount (in moles and mass) of sodium nitrate, but a different volume of solution, then we only have to change it:

So, the molarity of this solution is 0.1701 M.
Since the container of the gas is rigid, the volume of the gas will remain constant. Therefore, when the number of particles were decreased in half then the pressure will also be half of the original given they both are subjected to the same temperature.
PV = nRT
V, T and R are constants so they can be lumped together to a constant k.
P/n = k
P1/n1 = P2/n2
since n2 = n1/2
P1/n1 = P2/<span>n1/2</span>
P2 = P1/2
the great size or extent of something