Chemistry Midterm 1

Pure substance

Matter with a fixed constant composition, and any sample will have identical properties. Can be an element or a compound.

Element

A substance made up of only one atom that cannot be broken down chemically

Compound

A pure substance composed of two or more elements chemically bonded in fixed ratios. Can be broken down into simpler substances by chemical processes. 

Mixture

A non-pure physical combination of two or more substances that can be separated by physical means. 

Homogeneous mixture

uniform composition (solutions)

Heterogenous mixtures

non-uniform compositions (sand+water, salad)

Chemical changes

Convert matter from one chemical form to another. Produces a new substance and concurrent with rusting or burning. 

Physical changes

convert matter from one physical form to another without changing chemical composition. (melting, dissolving both change form but not identity)

Chemical properties

Describes a substances ability to engage or not engage in a chemical change. Describes the reactivity (flammability, acidity)

Physical properties

characteristics of matter that don’t involve changes to chemical composition. Color and density are both observable without identity change. 

Law of conservation of Mass

Mass is neither created nor destroyed in chemical or physical changes; total mass before = total mass after

Standard Sig fig rules

1. All non-zero digits are significant (e.g., 12.3 has 3 significant figures).

2. Zeros between non-zero digits are significant (e.g., 101 has 3 significant figures).

3. Leading zeros are NOT significant (e.g., 0.005 has 1 significant figure).

4. Trailing zeros are significant ONLY if a decimal point is present (e.g., 25.0 has 3 significant figures, but 2500 without a decimal has 2).

Multiplication and division sig figs

The answer has the same number of significant figures as the number with the fewest significant figures in the problem

Addition and subtraction sig figs

The answer is rounded to the fewest number of decimal places among the numbers in the problem. 

Why are sig figs important and what do they show?

They show the precision and uncertainty of a measurement and they show whether data is precise enough to support a claim.

How do we convert units?

Dimensional analysis employing conversion factors and canceling units

Density formula?

DMV triangle! Or, density = mass/volume

Law of definite proportions

All samples of a definite pure compound contain the same elements in the same proportion by mass.

A compound of 2 different samples have to always have the same ratio of elements by mass. 

Law of multiple proportions

When 2 elements react to form more than one compound, a fixed mass of 1 element reacts with the mass of the other element in a ratio of small whole numbers.

If two elements form multiple compounds, the mass ratios of one element compared to the other are simple whole numbers.

Dalton’s Atomic Theory

(1) All matter made of atoms. (2) Atoms of one element are identical. (3) Atoms combine in whole-number ratios to form compounds. (4) Atoms rearrange in reactions, not created/destroyed.

Parts of the nuclear atom (protons and electrons with charges)

An atom has a small, dense, positively charged nucleus of protons and neutrons, surrounded by a vast, mostly space filled with negatively charged electrons in orbitals

Where does an atoms mass lie? What determines atom’s size?

The nucleus contains almost all the atom's mass, while the electrons determine the atom's size

What is a nuclide symbol?

Format: _Z^A X ^charge

Z = atomic number

A = mass number

How to find neutrons from nuclide symbol?

Neutrons = mass number – atomic number.

What determines atomic number?

Atomic number = protons

What determines mass number?

Mass number = protons + neutrons

What determines charge

Charge = protons – electrons.

How to find average atomic mass?

multiply the mass of each isotope by its relative abundance (expressed as a decimal) and then add these products together. The formula is: 

Average Atomic Mass = (mass₁ × abundance₁) + (mass₂ × abundance₂)

Alkali Metals

group 1, soft, silver, water reactive, +1 cations

Alkaline Metals

group 2, soft, silvery white, semi-water reactive, +2 cations

Transition Metals

groups 3-12, harder, shiny, good thermal insulators, good electrical conductivity, high melting points, cations

Halogens

Diatomic molecules, gas at top, liquid in middle, solid at bottom, form salts with metals

Noble gases

monatomic gases, highly inert (unreative)

Nonmetals reactivity

Up on a group, right on a period

Metals Reactivity

Down a group, Left on a period

Molecular formula

will tell what atoms are in a compound and how many there are of each

Empirical formula

will show the smallest whole number ratio of atoms in a compound

Ionic compounds

Form cations + anions, metal and a non-metal, high melting points, conductive when dissolved. Have three-dimensional crystal lattice structure where positively charged ions (cations) and negatively charged ions (anions) are held in a regular, repeating pattern by strong electrostatic attractions

Molecular compounds

Non-metals bonded covalently, share electrons, can form isomers, lower melting/boiling points

Formula mass

for ionic compounds, it’s the sum of their atomic masses. 

Average mass of one “unit” of a given chemical formula

Molecular mass

for covalent compounds, it’s the sum of atoimic masses in the molecule

Mass of one mole of a molecule that is numerically equal to the formula mass in amu

Formula vs molecular mass Similarity and difference

Both are calculated identically by summing the atomic masses from the periodic table to find the total mass of the formula unit or molecule in atomic mass units (amu).

The key difference lies in the nature of the compound: ionic compounds do not exist as discrete molecules, necessitating the term "formula mass," whereas covalent compounds do, allowing for the term "molecular mass". 

What is a mole?

1 mol = 6.022×10²³ particles (atoms, molecules, ions)

how to convert between mass and moles

Mass ↔ Moles (use molar mass)

how to covert between moles and atoms

Moles ↔ Atoms (use Avogadro’s number)

Percent composition formula?

% element = (mass of element ÷ total molar mass) × 100.

Empirical vs. molecular formula?

Empirical = simplest ratio, Molecular = actual composition (multiple of empirical).

How to find empirical formula from % composition?

Convert % to grams → to moles → divide by smallest mole → get whole- number ratio.

What is mole fraction?

Mole fraction = moles of component ÷ total moles.

What is mass fraction?

Mass fraction = mass of component ÷ total mass.

How to calculate solution concentration (molarity)?

M = moles of solute ÷ liters of solution.

Dilution formula?

M₁​V₁​=M₂​V₂​.

Ionic compounds (naming)

name the cation, name the anion with an -ide. (NaCl - Sodium Chloride)

Transition Metals (naming)

name the cation, name the anion with an -ide, include roman numerals after multiple charge ion.

Molecular compounds (naming)

covalently bond, must specify how many with prefixes

mono

one

di

two

tri

three

tetra

four

penta

five

hexa

six

hepta

seven

octa

8

nona

9

deca

10

ammonium

NH₄⁺

hydroxide

OH^-

acetate

CH₃COO^-

cyanide

CN^-

carbonate

CO₃^2-

nitrate

NO₃^-

sulfate

SO₄^2-

phosphate

PO₄^3-

permanganate

MnO₄^-

Length

meters (m)

Mass

kilograms (kg)

Time

seconds (s)

Temperature

Kelvin (K)

k_ (kilo)

10^3

c_ (centi)

10^-2

m_ (milli)

10^-3

µ_ (micro)

10^-6

n_ (nano)

10^-9

1m^3

1000L

1L

1000cm^3

1mL

1cm^3

Volume of a cube

V=s^3 (one side cubed)

Area of a square

A=s² (one side squared)

Area of a circle

A=πr²

Celcius to Kelvin

K = °C + 273.15

Fahrenheit to celcius

°C = (°F - 32) × 5/9

Molar Mass

Mass of ONE MOLE.

M=m/n → M is g/mol, m is mass of the substance, n is the number of moles.

Average Atomic Mass

Mass of ONE ATOM

amu = number of atoms x periodic mass.

Molar mass vs AMU

Both are numerically the same

AMU - Mass one ONE ATOM

Molar Mass - mass of ONE MOLE