Question

What are ways you can determine the amount of a substance that is unknown? Concentration, depth, volume and density? Just formulas like M= D/V ect... Please give an example and explain for 5 star rating.

Answer 1

One way of determining an unknown substance quantity is concentration, for example, in an aqueous solution. If we are given a solution of NaF with a 0.05 M concentration, we can get the quantity of NaF in solution with its molar mass and the volume of solution. For example, for a 500 mL solution:

Molar Concentration = moles / Liters

0.05 M = 0.05 mol / L

500 mL = 0.5 L

Moles of NaF = 0.05 mol/L * 0.5 L = 0.025 moles

Molar mass of NaF is 42 g/mol

0.025 moles of NaF * (42 grams of NaF / 1 mol of NaF) = 1.05 grams of NaF

Another way to measure unknown quantities is by mass concentration, for example, if we are given a flow of 100 kg/hr of a benzene and toluene mixture with a concentration of 45% wt benzene and rest is toluene, we can say we have 45 kg/hr of benzene and 55 kg/hr of toluene.

For density, simple example we can use is water. Imagine we have 100 mL of water, and we want to know its mass. We know that water density is 1 kg/L and:

density = mass / Volume

1 kg/L = mass / 0.1 L

mass = 0.1 kg = 100 g

Another way is to use molality, which is:

molality = moles of solute / kg of solvent

We have, for example, 1L of a H₂SO₄ 7.69 molal solution, with a density of 1.3 g/cm³, then how would we get the amount of sulfuric acid

We would get first, the whole mass of solution with the volume and density:

1.3 g/cm3 * (100cm /1m)³ * (1 m³ / 1000L) = 1300 g/L

If we have 1 L of solution, we then have 1300 grams of solution, or 1.3 kg

Then we use molality definition:

7.69 = moles of solute / kg of solvent

For moles of solute (molar mass in kg/mol):

moles of solute = mass of solute / molar mass of H2SO4

moles of solute = mass of solute / 98

For kg of solvent:

kg of solvent = mass of solution - mass of solute

kg of solvent = 1.3 kg - mass of solute

Substituting:

7.69 = [mass of solute / 0.098] / [1.3 - mass of solute]

Isolating for mass of solute, or H2SO4:

9.997 - 7.69(mass of solute) = mass of solute / 0.098

0.9797 - 0.7536(mass of solute) = mass of solute

0.9797 = 1.7536 (mass of solute)

mass of solute = mass of H2SO4 = 0.5587 kg = 558.7 grams of H₂SO₄