Solutions and Concentration: NMAT Chemistry Review

Why Solutions Matter in NMAT Chemistry

Many chemistry questions in the NMAT revolve around real-world mixtures: saltwater, IV fluids, household acids, cleaning solutions, and laboratory reagents. A “solution” is a homogeneous mixture where one substance (the solute) is dispersed uniformly in another (the solvent). Understanding solutions helps you answer questions about dilution, reaction stoichiometry in aqueous systems, acid–base calculations, colligative properties, and even electrochemistry.

In NMAT-style problems, you are often given concentrations, volumes, or masses and asked to determine an unknown. The fastest way to succeed is to know the key concentration units, when to use each, and the common shortcuts (like dilution equations and mole relationships).

Basic Terms: Solute, Solvent, and Solution

• Solute: the substance being dissolved (e.g., NaCl in saltwater).
• Solvent: the dissolving medium, usually present in larger amount (e.g., water in saltwater).
• Solution: the homogeneous mixture formed.

Solutions can be aqueous (solvent is water) or non-aqueous (solvent is something else, like ethanol). Solutes may be solids, liquids, or gases. For NMAT, aqueous solutions are the most common.

How Dissolving Works: “Like Dissolves Like”

Solubility depends on the nature of solute and solvent:

• Polar solvents (water) dissolve polar/ionic solutes (salt, sugar, acids).
• Nonpolar solvents (hexane) dissolve nonpolar solutes (oils, waxes).

This is summarized by the rule “like dissolves like”. Water dissolves ionic compounds because its polarity stabilizes ions through ion–dipole interactions. For molecular solutes, hydrogen bonding and dipole interactions often explain solubility trends.

Solubility, Saturation, and Temperature Effects

Solubility is the maximum amount of solute that dissolves in a given amount of solvent at a specific temperature (and pressure, for gases).

• Unsaturated solution: can dissolve more solute.
• Saturated solution: contains the maximum dissolved solute at that condition.
• Supersaturated solution: contains more solute than equilibrium allows; unstable and can crystallize if disturbed.

For many solids, solubility increases with temperature. For gases in water, solubility typically decreases with temperature (warm soda loses CO₂ faster). This concept can appear in conceptual NMAT questions.

Concentration: What It Means and Why Units Matter

Concentration tells you how much solute is present relative to the solution or solvent. Different units are used depending on context:

• Molarity (M): moles solute per liter of solution.
• Molality (m): moles solute per kilogram of solvent.
• Mass percent (% w/w): mass solute per mass solution × 100.
• Volume percent (% v/v): volume solute per volume solution × 100.
• Mass/volume percent (% w/v): grams solute per 100 mL solution.
• Parts per million (ppm) and parts per billion (ppb): very dilute concentrations.
• Mole fraction (X): moles of a component divided by total moles in mixture.

NMAT questions commonly use molarity and dilution, but can also include percent solutions and ppm for “water quality” or “trace solute” contexts.

Molarity (M): The Most Common NMAT Concentration Unit

Molarity is defined as:

M = (moles of solute) / (liters of solution)

Key reminders:

• Volume is in liters, not mL. Convert mL to L by dividing by 1000.
• Molarity uses solution volume, not solvent volume.
• Molarity changes with temperature because solution volume can change with temperature.

Example logic: If you dissolve 0.50 mol NaCl and make the final solution volume 1.0 L, the solution is 0.50 M.

Making Solutions: From Mass to Molarity

A classic NMAT skill is converting grams of solute into molarity.

1. Convert grams → moles using molar mass.
2. Convert volume to liters.
3. Use M = moles/L.

For example, if 5.85 g NaCl (molar mass ≈ 58.5 g/mol) is dissolved to make 0.500 L solution:

• moles = 5.85 / 58.5 = 0.100 mol
• M = 0.100 / 0.500 = 0.200 M

Even if the test does not ask you to show steps, having this mental checklist reduces errors under time pressure.

Dilution: The Fastest High-Yield Formula (M1V1 = M2V2)

Dilution problems are extremely common. If you dilute a solution, the moles of solute stay constant (assuming no reaction), but the volume increases, so concentration decreases.

The shortcut equation:

M₁V₁ = M₂V₂

• M₁ = initial concentration
• V₁ = volume taken from stock solution
• M₂ = final concentration
• V₂ = final total volume after dilution

Important: volumes must be in the same units (mL and mL is fine; L and L is fine).

Concept check: If you add water, V increases, so M decreases.

Serial Dilutions and Dilution Factors

A serial dilution is done in multiple steps. A simple way to handle these problems is through a dilution factor:

• Dilution factor = (final volume) / (aliquot volume)
• Final concentration = initial concentration / dilution factor

For serial dilutions, multiply dilution factors across steps.

Example idea: Taking 10 mL to make 100 mL total is a 10× dilution (factor 10). Doing that twice gives 100× overall.

Percent Solutions: % w/w, % v/v, and % w/v

Percent concentrations show up in medical and household contexts.

• % w/w = (mass solute / mass solution) × 100
• % v/v = (volume solute / volume solution) × 100
• % w/v = (grams solute / 100 mL solution)

A common NMAT interpretation: A “5% (w/v) glucose solution” often means 5 g glucose per 100 mL solution. So in 500 mL, glucose mass = 25 g.

Also common: “70% alcohol” typically refers to volume percent, depending on context. In many exam problems, the unit is implied; use the given information to infer the correct type.

Parts per Million (ppm) and Parts per Billion (ppb)

These units are used for very dilute solutions (environmental chemistry, contaminants).

• ppm ≈ mg solute per liter of water (mg/L) for dilute aqueous solutions
• ppb ≈ μg solute per liter (μg/L)

Why the approximation works: 1 L of water has a mass close to 1 kg at room temperature, so mg/L is near mg/kg.

Example: 3 ppm fluoride in water means ~3 mg fluoride per liter.

Molality (m): When Mass Matters More Than Volume

Molality is:

m = (moles of solute) / (kg of solvent)

Unlike molarity, molality does not change with temperature because it depends on mass, not volume. Molality is especially important in colligative property calculations (boiling point elevation, freezing point depression), though NMAT may test it conceptually or with simple plug-in problems.

Mole Fraction (X): Concentration in Terms of Moles

Mole fraction of component A:

X_A = (moles of A) / (total moles of all components)

Mole fraction is dimensionless and is useful in vapor pressure and some colligative property contexts (Raoult’s law). NMAT questions may ask for a mole fraction given moles (or masses converted to moles) of two substances.

Converting Between Concentration Units

Converting between molarity, mass percent, and ppm can require density (because mass and volume are linked by density). If density is not given, the problem often assumes dilute aqueous solution (density ~ 1.00 g/mL).

Common conversion strategies:

• Mass percent → grams solute: assume 100 g of solution, then compute solute mass.
• % w/v → grams solute: use the definition “g per 100 mL”.
• ppm → mg/L (dilute water): 1 ppm ≈ 1 mg/L.
• Molarity → grams per liter: multiply M by molar mass (g/mol).

If a question seems impossible without density, look for a clue: “aqueous,” “dilute,” or values that suggest the 1 g/mL approximation.

Mixing Solutions: Same Solute, Different Concentrations

Another frequent NMAT pattern: mixing two solutions of the same solute. The key is to add moles:

1. Compute moles from each solution: moles = M × V (in liters).
2. Add moles together.
3. Add volumes together (assuming volumes are additive, as most exam problems do).
4. New molarity = total moles / total volume.

This is essentially a mass balance approach. It works reliably and avoids memorizing extra formulas.

Solution Stoichiometry: Reactions in Aqueous Form

NMAT often tests stoichiometry in solution, where reactants are given in molarity and volume rather than grams. The procedure:

1. Convert each reactant to moles using moles = M × V (L).
2. Use the balanced equation to determine the limiting reactant.
3. Find moles of product formed or moles of excess remaining.
4. If needed, compute concentration of product by dividing moles by final volume.

This merges two high-yield topics: concentration and limiting reactants. Be careful with units and balanced coefficients.

Solubility Rules and Electrolytes: Strong vs Weak

Many solutions in NMAT problems contain ions. When an ionic compound dissolves, it may dissociate into ions:

• Strong electrolytes (e.g., NaCl, strong acids/bases): dissociate almost completely.
• Weak electrolytes (e.g., acetic acid, ammonia): partially ionize.
• Nonelectrolytes (e.g., glucose): dissolve but do not form ions.

This matters when interpreting conductivity, colligative properties (number of particles), and acid–base equilibrium. While detailed electrolyte calculations may be advanced, conceptual recognition is important.

Common NMAT Pitfalls and Quick Tips

• Forgetting to convert mL to L in molarity calculations.
• Using solvent volume instead of solution volume when computing molarity.
• Applying M1V1=M2V2 when a reaction occurs. Dilution formula assumes moles are conserved (no reaction).
• Ignoring significant figures in final answers; follow what the problem expects.
• Mixing up % w/v and % w/w. Remember: w/v is grams per 100 mL solution.
• Not checking reasonableness: dilution should decrease concentration; adding solute should increase it.

High-Yield Mini Summary

• A solution is a homogeneous mixture of solute + solvent.
• Molarity (M) = moles solute / L solution; most common in NMAT.
• Dilution uses M1V1 = M2V2 when no reaction occurs.
• Percent solutions are common: % w/v = g per 100 mL solution.
• ppm ≈ mg/L for dilute aqueous solutions; ppb ≈ μg/L.
• For mixing solutions, add moles then divide by total volume.
• For solution stoichiometry, convert M and V to moles, then use balanced equations.