A solution is made containing 8.4 g of potassium nitrate per 125 g of
water.
What is the weight/weight % or
percent by mass of the solute?

The percent by mass : 6.3%

The concentration of a substance can be expressed in several quantities such as moles, percent (%) weight/volume,), molarity, molality, parts per million (ppm) or mole fraction. The concentration shows the amount of solute in a unit of the amount of solvent.

mass of potassium nitrate = 8.4 g⇒solute

mass of potassium nitrate + mass of water = 8.4 g + 125 g =133.4 g

Percent by mass of the solute :

where's the diagram?

answer is: naturally occurring solid, inorganic compound or element is mineral.

minerals usually have crystalline form, that means than crystal constituents (atoms, molecules or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions.

for example magnetite is a mineral of iron oxide (fe₂o₃).

calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (caco₃).

dolomite is an   carbonate mineral composed of calcium magnesium carbonate (camg(co₃)₂).