Chem Unit 7 Solutions Test

Solution

A homogeneous mixture of a solute dissolved in a solvent

Solute

The substance being dissolved in a solution

Solvent

The substance doing the dissolving (usually H₂O)

Why does a solute dissolve?

A solute dissolves if it is more attracted to the solvent than to itself

Molarity (M)

Moles of solute per liter of solution; M = mol/L

Molarity formula

M = moles of solute ÷ liters of solution

How to convert mL to L

Divide by 1000 (1000 mL = 1 L)

Molarity as a conversion factor

0.35 mol KF / 1 L soln OR 1 L soln / 0.35 mol KF

Dilution equation

M₁V₁ = M₂V₂ (molarity × volume of stock = molarity × volume of dilute)

Stock solution

A solution with a high concentration used to make lower concentration solutions

Dilution

Adding solvent (H₂O) to a more concentrated solution to make it less concentrated

Steps to make a stock solution

1) Determine desired molarity 2) Mass out required solute 3) Add solute to flask 4) Add some solvent to dissolve 5) Fill to desired volume mark, mix until homogeneous

Correct order for making a stock solution (5 steps)

1) Weigh solute 2) Pour into volumetric flask 3) Add some water to dissolve 4) Fill to volumetric mark 5) Swirl until homogeneous

Saturated solution

A solution where the solid is dissolved as much as possible — no more solute can dissolve

Unsaturated solution

A solution where the solid isn't dissolved to its full potential — more solute can still dissolve

Supersaturated solution

A solution that contains more dissolved solute than a saturated solution at that temperature (above the curve on a solubility graph)

Saturation point

On the solubility curve — the exact point where the solution is saturated

Unsaturated on a solubility graph

Below the curve

Saturated on a solubility graph

Above the curve

Saturation point on a solubility graph

On the curve

Temperature & solubility of solids

As temperature increases, solubility of solids increases (direct relationship)

Temperature & solubility of gases

As temperature increases, solubility of gases decreases (inverse relationship)

Why does temp increase solubility of solids?

Higher temp → particles move faster → more collisions between solute and solvent → breaks forces of attraction → more solute dissolves

Why does temp decrease solubility of gases?

Higher temp → more collisions → gas particles gain energy and escape the liquid → less gas stays dissolved

Coulombic attraction and solubility

Smaller ions (like Na⁺, Cl⁻) have greater coulombic attraction, making them harder to dissolve; larger ions (like K⁺, Br⁻) have weaker attraction, making them easier to dissolve

NaCl vs KBr solubility

NaCl stays nearly the same with temperature change (strong coulombic attraction); KBr increases more with temperature (weaker coulombic attraction)

Double Replacement Reaction (molecular equation) — steps

1) Determine products using D.R. pattern, form neutral compounds 2) Determine phase labels using solubility table 3) Balance using coefficients

Double Replacement — if ALL compounds are (aq)

No reaction (NRxN)

Complete Ionic Equation — steps

1) Break up all (aq) compounds into ions with charges 2) Keep (s), (l), (g) together (not dissolved) 3) Must still be balanced

Spectator ions

Ions that do NOT participate in the formation of the solid, liquid, or gas — they appear the same on both sides of the equation

Net Ionic Equation — steps

1) Rewrite complete ionic equation without spectator ions 2) Must still be balanced

Net ionic equation example

CO₃²⁻(aq) + Ca²⁺(aq) → CaCO₃(s)

Ionic compounds in solution

Break apart into individual ions (e.g., (NH₄)₂CO₃ → 2NH₄⁺ + CO₃²⁻)

Polyatomic ions

Do NOT break apart into individual atoms when dissolved

Beer's Law

Absorbance is directly proportional to concentration; higher concentration = higher absorbance

Beer's Law equation (line of best fit)

y = 0.4083x + 0.0019 (y = absorbance, x = concentration)

How to find unknown concentration using Beer's Law

Plug the unknown absorbance into the Beer's Law equation and solve for x (concentration)

Dilute solution

A solution with a low concentration of solute

Concentrated solution

A solution with a high concentration of solute

When adding water to a NaCl solution…

The salt concentration decreases

What do you need to calculate molarity?

Moles of solute AND total volume of solution (in liters)

Thermal pollution & gas solubility

As water temperature increases (from heated water returned to ecosystems), gas solubility decreases → less dissolved oxygen → harms aquatic life

Thermal pollution & solid solubility

As water temperature increases, solid solubility increases → more particles/sediments dissolve and erode → harms ecosystem