November 28 2010 - Percent Composition

The percent composition of a compound is a relative measure of the mass of each element present in the compound.  


To calculate the percent composition (percentage composition) of a compound
1.Calculate the molecular mass (molecular weight, formula mass, formula weight), MM, of the compound
2.Calculate the total mass of each element present in the formula of the compound
3.Calculate the percent compositon (percentage composition): % by weight (mass) of element
    = (total mass of element present ÷ molecular mass) x 100


Eg.
Calculate the molecular mass (MM):MM = (2 x 22.99) + 32.06 + (4 x 16.00) = 142.04


Calculate the total mass of Na present:2 Na are present in the formula, mass = 2 x 22.99 = 45.98


Calculate the percent by weight of Na in Na₂SO₄:%Na = (mass Na ÷ MM) x 100 = (45.98 ÷ 142.04) 
x 100 = 32.37%
Calculate the total mass of S present in Na₂SO₄:1 S is present in the formula, mass = 32.06


Calculate the percent by weight of S present:%S = (mass S ÷ MM) x 100 = (32.06 ÷ 142.04) x 100 
= 22.57%


Calculate the total mass of O present in Na₂SO₄:4 O are present in the formula, mass = 4 x 16.00 = 64.00


Calculate the percent by weight of O in Na₂SO₄:%O = (mass O ÷ MM) x 100 = (64.00 ÷ 142.04) x 100 = 45.06%


The answers above are probably correct if %Na + %S + %O = 100, that is,     32.37 + 22.57 + 45.06 = 100

November 19, 2010

Mole Conversions


1. 6.022 * 10^23 particles = 1 mole
a) particles to moles
Particles / (6.022*10^23) = moles
Ex. (3.01*10^24 particles) * (1mole / 6.022*10^23 particles) = 5.00 moles with significant figures
b) moles to particles
moles * 6.022*10^23 = particles
Ex. 0.75 moles * (6.022*10^23 molecules) / 1mole = 4.5 * 10^23 molecules with significant figures
c) grams to moles
mass of something * (1mole / atomic mass)
Ex. 3.45g Carbon to moles
3.45g * (1mole / 12.0g [Carbon's atomic mass]) = 0.288 moles Carbon
Ex. 6.2g of MgCl2 to moles
6.2g * (1mole / 95.3g [MgCl2 atomic mass])  = 0.0650 moles MgCl2

November 1 2010 - Density

A materials density is defined as its mass per unit volume.  It is, essentially, a measurement of how tightly matter is compacted together.  

To calculate the density of an object, you take the mass and divide it by the volume.  The equation will look like this : P=M/V  where P is the Density, M is the mass and V is the volume. 

The unit for solids are g/Cm³ and the unit for liquids are g/mL

1 Cm³ of water = 1mL

The density of water is 1.0g/mL or 1000g/L

If the density of an object is greater than the density of a liquid, then it will sink

If the density of an object is less than the density of a liquid, then it will float.

=) DENSITY VIDEO!!!!!!!!!