Wednesday, November 18, 2009

Exam Time!!

So basically we reviewed the homework and got the mid-term review sheets.

here are a few sites if yaaa need extra practice/study material!!
-molar mass
-calculating percentage
-molar equations

Friday, November 13, 2009

Empirical Formulas

empirical formulas are pretty much just the simplest version of any chemical formula.

Molecular Empirical
P4H10 - - - - - P2H5
C10H22 - - - - - C5H11
C6H18O3 - - - - - C2H6O
N2O4 - - - - - - NO2

empirical formulas the simplest whole number ratios in a compound

molecular atoms show the actual atoms and bonds in a compound

finding empirical formulas:
a sample of an unknown compound in analyzed and found to contain 8.4g of carbon, 21g of hydrogen and 5.1g or oxygen. find the empirical formula

element - atomic mass- mass - moles - smallest divided - smallest ratio
carbon 12 8.4 .7 2.187 2
hydrogen 1 2.1 2.1 6.56 7
oxygen 16 5.1 .32 1 1

the empirical formula = C2H7O

comprende? thats pretty much all we did, it was a short class today

for a fun worksheet, click here!

Tuesday, November 10, 2009

Percentage Mass of Elements in Compounds

The only formulas you need are:
Molar Mass: G/Mol and Molar Volume: L/Mol
That is basically it and now here are some examples to practice.

EXAMPLE 1:

Calculate the percentage of each element in C3H8.
Step 1: Formula used should be molar mass: G/Mol.
Step 2: Find the molar mass of each element.
Carbon: 3(12.0) + Hydrogen: 8(1)= 44g/mol
To find the percentage divide one of the element's molar mass by the total molar mass of the compound.
Step 3: Carbon: 36g/mol/44g/mol= .818 x 100= 82.0%. Hydrogen: 8g/mol/44g/mol= .181 x 100=18%

EXAMPLE 2:
Find the percent mass of XeF2
Step 1: Use formula molar mass: G/Mol
Step 2: Find molar mass of each element.
Xenon: 131.1 + Flourine: 2(19.0)= 38 = 169.1
Step 3: Divide individual molar mass by total molar mass.
Xenon: 131.1/169.1= .775 = 78% Flourine: 38/169.1=.2247 = 22%

Friday, November 6, 2009

Some More Equations

remember:













Example:





The density of water is 1.00 g/l. Find the volume of water occupied by 1.6 x 1024 molecules of H2O





we will have to convert molecules--->moles---->mass.


A 4.00 mL sample of an unknown substance is known to contain 0.228 mols. Its molar mass is 196.967 g/mol. What is it's density?
we better get a good mark on this, it took a long time.


Thursday, November 5, 2009

Density and Moles

Density --> MASS per unit VOLUME
  • D=M/V
  • M=D/V
  • V=M/D
Density of Gases at STP
Density of Solids and Liquids

& remember...

Monday, November 2, 2009

Molar Volume Lab

today we did the molar volume lab!

for homework we made a procedure for the experiment that mr doktor showed us last class and today we carried it out, after going over some questions we had for homework.

in this lab we start by filling the sink with water and measuring the mass of the lighter. once we do that, we place the 100mL graduated cylinder under the water and let all the air bubbles flow out so it is full of water (upside down). then we hold the cheap, NONSEALED (mr doktor didnt spend much on the lighters :P) lighters under the opening in the graduated cylinder and hold the button down so the butane flows out into it up to 10mL. we then measure the mass of the lighter after the butane is released. this is where most problems occured due to the fact that when we released the butane, the water flowed into the lighter, throwing the mass off. using the difference in mass we calculated the molar volume of butane. even though it should have come out to 22.4 L per mole, we came out with 2.9 L. Yeah, just a little bit off. but hey we got to light stuff and see bubbles rise so it was fun and we got to practice some important calculations.

thats pretty much all we did today, as the class was shortened and the lab took up the whole time so there was no lesson otherwise. till next time "show your work and prosper"

wow sorry that was a painfully cheesy ending

Sunday, November 1, 2009

ATOMS AND MOLES

1) For monoatomic elements:
a molecule= an atom

2) Diatomic elements
A MOLECULE: AN ATOM:
Cl2. Cl

3) Molecules of Compounds
2 Hydrogen atoms and 1 oxygen atom create water which is one molecule.

FORMULA FOR ATOMS AMD MOLES
6.02x10^23molecules/ 1 mol OR 1 mol/ 6.02x10^23

Examples:
how many molecules in 0.25mol of CO2?
0.25mol x 6.02x10^23 molecules= 1.51x10^23

How many moles in 5.1772x10^24 molecules of H2O?
5.1772x10^24 molecules/ 6.02x10^23= 8.6mol

For more practice here is a printable practice worksheet.
www.emp.byui.edu/PECKK/chem105/molsheet.htm