RR: Write generic equations to convert: massmoles, particlesmoles, & massparticles

RR: How does the percentage of an ingredient affect a total product?(Think of things that have ingredients.)

Percent Composition• In chemistry, percent composition is the amount of an

element relative to the amount of the entire molecule or compound.

% = OR

% composition (of element) =

RR: Why is it important to consider %composition in daily life?https://www.youtube.com/watch?v=DxR4PqPlgzQ

RR: Calculate the %composition when 13.3g Fe combines completely with 5.7g O.

Can I figure out the %composition without the mass of the compound given?•Remember that I can get the molar mass of the

compound & the molar masses of each element in the compound.• So, I can use the molar mass instead of actual

(experimental) mass to get the %composition (since the mass of each is relative to 1 mole of the compound).

Percent Composition using Molar Mass

%composition =

RR: What is the %composition of Ag2O?

Percent Composition as a Conversion Factor• If you have %composition, you can determine the

mass of each element in a compound.•Use %composition as a conversion factor.

If the %composition of FeO is 70% Fe and 30% O, what is the mass of Fe and O in FeO?

If I have the %composition of a formula, how could I determine what the formula is?

How can we use %composition to get a chemical formula?Ex) If I have 20% H and 80% C, what could the formula be for this combination of elements?• Start with what we know:

• 80• These values added together make up 1 mole of the

compound.

• So we can find empirical mass of each element by assigning each percentage mass units (grams).•But we still need to get from the mass of each to a

formula with a certain number of moles of each element in it.•How do we get from mass to moles?

•Use MOLAR MASS to get moles of each element.

Using Molar Mass…20g H mol H80g C mol CNow divide each by the smallest number to get the smallest ratio.

This means the ratio of H to C is 3:1.We can write the empirical formula as H3C

Empirical Formula• The formula we just determined is the empirical

formula – the formula with the smallest whole-number ratio of atoms in a compound.

RR: Pick a problem from % Composition WS to review.

RR: What is an empirical formula?

Steps to get Empirical Formula1. Assign grams to the percent composition values.2. Convert grams to moles using molar mass.3. Divide each value by the smallest number of moles calculated.4. Round to the nearest whole number.5. Assign the rounded moles of each as the subscripts in the empirical

formula.

RR: What is the first step in getting an empirical formula?

RR: Determine the empirical formula of a compound consisting of 38.7% carbon, 9.75% hydrogen, and the rest oxygen.

Molecular Formula A molecular formula is the same as or a multiple of the empirical formula, and is based on the actual number of atoms of each type in the compound.

Steps to get Molecular Formula1. Get the empirical formula.2. Calculate the empirical formula mass.3. Divide the molecular formula mass by the empirical

formula mass.

4. Multiply the empirical formula by the ratio calculated to get the molecular formula.

RR: What is the only conversion factor used in determining an empirical and molecular formula?