Chem Exam Gr 11 - Unit 3

how precise shouldn measurments be?

always report one uncertain digit

Accuracy

- how close a measured value is to the actual (true) value

- accuracy is correct (bullseye)

Sig Digs

x/

Precision of a Group of Measurements

- how close measured values are to each other

- precision is repeating (same spot, may not be bullseye)

Decimal places

+-

when is precisoin used

+and -

same deciaml palces as least precise number

when is accuracy used

x and /

same SD as least accurate number

what numbers do not count for SD?

given/unchangable values

atomic mass

mass of an atom on a sclae of C-12 = 12u

symbol: m

unit: g

molecular mass

mass of a molecule
sum of atomic masses of theatoms

symbol: m

unit: g

formula mass

mass of one formula unit of a ionic compound

sum of atomic mases of atoms that make up formula unit

symbol: m

unit: g

mole

represents a quantity
def: quantity of a substance that contains avogadros number of units

symbol: n

unit: mol

avogadros number

6.02 × 10²³ particles/mol
the amount of particles in 12g of C-12
symbol: Na

molar mass

mass of one mole of a substance

symbol: M

unit: g/mol

formulas for mole, mass, and particles calculations

n = m / M
n = N / Na

composition (of a compound)

quantity (often by mass) of each element that makes up a compound (often expressed as %)

percentage composition by mass

mass of an element in a compound expressed as a % of the total mass or the compound

%Xtotal = (Mx / Mtotal) x 100%

reasons for % compsition > 100%

foreign substances
improper drying (only for anhydrous)

measurment errors

reasons for % compsition being too low

incomplete reaction
loss of product (limiting reagent) during transfer

—> loss of excess reagent MIGHT affect the percent composition

empirical formula

simplest formula ratio of a substance

—> derived from expiremental observations

melocular formula

actual formula of substance'

—> derived from EF and molar mass

what do coefficients mean in formulas

# of moles
mol ratio = coefficient ratio