December 4, 2009 (molecular formulas)

Finding Molecular Formulas
It is easy to do if you know the empirical formula and the Molar Mass.

If the empirical formula for a compound is CH2O and the molar mass is 90.0g/mol. Find the molecular formula.
Stock Solutions

• Are concentrated solutions from suppliers

12 M HCl
15 M HNO3
18M H2SO4

Makiki wants to make 200mL of 0.40M HCl. What volume of 10.0M HCl should she use?
V1= ? V2= 0.200L C2V2 = C1V1
C1= 10.0M C2= 0.40M (0.40M)(0.200L) = (10.0M) V1
V1 = 0.008L/ 8mL

Giving Directions - Dec2 Class

In today's class, we learned about which procedures we should use when it comes to concentrations. We also did a pre-lab about how much water we would need to dissolve Copper Chloride completely. We will be doing the lab this friday, Dec4.

Outline for experimental procedures:
- Find the mass you need

Johnny is asked to make 0.95M solution of K2S04. If he needs 300mL, what procedure should he use?
Con'c -> moles -> mass

Give directions to make 3.5 L of 4.5M NaOH

DILUTIONS OF SOLUTIONS
- When you add water, con'c decreases. If the volume is doubled, con'c is halved.

N1 = N2
C1V1=C2V2
Lauren adds 215.0 mL of water to 25.0 mL of 0.8M HCl. Find [HCl]
Janice adds water to 75.0 mL of 0.15M HF to a final volume of 310mL. Find the [HF]
Nicola dilutes 75.0 mL of 0.30M HNO3 to 0.10M. What is the final volume?

How much water did she add?
225mL-75mL = 150mL

Concentration- Nov. 30

In Tuesday's class we learned about concentration and its units.

Vocabulary:
Solution- A homogeneous mixture
Solute- The one present in smaller amount
Solvent- The one present in greater amount
Concentration- Amount of solute/Amount of solvent

Some units for Concentration:
g/mL, g/L, mg/L, mg/mL, ug/L

The most common (and useful) units are:
mol/L = Molarity = Concentration

Examples:
Stefano dissolves 108g of NaBr in enough water to make 300mL of the solution. What is the concentration?
Con'c = 108g/300 mL = 0.36g/mol [NaBr] = ? 0.300L = V

108g x 1 mol/108g M = 1.0mol/0.300L = 3.33mol/ L = 3.33M

Jayson wants to make 600.0mL of 0.60M CaCl2. What mass of solid CaCl2 is required?
V = 0.600L con'c ---> mol ---> mass
mol = M x L = 0.6m/L x 600L = 0.36mol x 111.1g/1mol = 39.996 or 40g

November 16th Class

On November 16th we learned about empirical formula's and how to find them.

Empirical Formula's
- give the whole number of ratio's of elements in a compound while molecular formula's give the actual number.

After learning what empirical formula's are we learned how to find them.

Finding Empirical Formula's

Example:
A sample of an unknown compound is analyzed and found to contain 8.4 g of carbon 2.1 g of hydrogen and 5.6 g of oxygen.

* The easiest way to answer this question is to organize the information that is given into a chart format.


C2H6O

If the ratio ends in a decimal of... 0.5 then multiply by 2.
If the ratio ends in a decimal of... 0.33 or 0.66 then multiply by 3.
If the ratio ends in a deciaml of... 0.25 or 0.75 then multiply by 4.
If the ratio ends in a decimal of... 0.2, 0.4 or 0.6 then multiply by 5.

Nov.12 Class

In the beginning of class we reviewed 10-2 questions and everything up to the mole conversion is on the midterm.

molar mass -> g/mol

molar volume -> L/mol -> mL/mol

Example:
A sample of an unknown gas contains 0.0554mol and occupies a volume of 602.0mL. Determine the molar volume.

Percent Mass of Elements in Compunds

Find the % carbon by mass in Glucose. (C6H12O6)
C = 6(12.o) + H = 12(1.0) + O = 6(16.0) = 180 g/mol
% of Carbon = 72g/180g = 0.4 -> 40%
% of Hydrogen = 12g/180g = 0.0666... -> 7%
% of Oxygen = 96g/180g = 0.5333... -> 53%

Percent Composition
- means the % mass of each element in a compound.

Find the composition of NO3
N = 1(14.0) + O = 3(16.0) = 62.0g/mol
% of Nitrogen = 14g/62g = 0.228 -> 23%
% of Oxygen = 48g/62g = 0.77 -> 77%

Finding the mass of an element in a given mass of a compound.

Find the mass of Carbon contained in a 30.0g sample of CO3.
(First find the percent. Then take the percentage as a decimal and multiply it by the given amount.)
C = 1(12.0) + O = 3(16.0) = 60g/mol
% C = 12g/60g = (0.200) (30.0) = 6g
% O = 48g/60g = (0.800)(30.0) = 24g

Find the mass of K,C, and O contained in K2CO3 if the sample is 500.0g
K = 2(39.1) = 78.2 + C = 12 + O = 16(3) = 48 = 138.2g/mol
% of K = 78.2/138.2 = 56.6% -> 0.566(500g) = 283.0g
% of C = 12/ 138.2 = 8.7% -> 0.087(500g) = 43.5g
% of O = 48/138.2 = 34.7% -> 0.347(500g) = 173.5g
Total = 500.0g

Density & Moles - Continued: Nov9 Class

Today in class, we drew out the flow chart for all the calculations we could do with this unit of moles. We did 3 examples and the last one is one made up.

Example: 1.25L of an unknown gas has a mass of 3.47g. What is the molar mass if it is at STP?
Example 2: A 5.00g sample of solid lead contains 0.247 mol of Pb. Calculate the density of Lead.

Example 3: 250mL of a gas which is known to contain one sulfur atom and an unknown number of fluorides has a mass of 1.63 g at STP
SFx
Step 1: Find the molar mass (g/mol)

Step 2: Find the chemical forumla


Example 4: A 8.00mL sample of (solid) Tin contains 0.632 mol of Sn. Calculate the density of Tin.


We spent the last 20-25 minutes of class, finishing questions #11-30 on the worksheet.

Density & Moles- Nov 5

On Thursday class we learned about density of gases at STP and how to convert into mole from different units.

Density ---> mass per unit volume


Density of gases at STP
-1 mole of gas

Calculate the density of O2 at STP


Example:

A mystery gas has a density of 1.696g/L at STP. It is a diatomic element. Identify the gas.
MM = (1.696 g/L)(22.4 L/mol) = 38 g/mol--->19 g/mol---> Flourine, F2


The density of Boron (solid) is 2.34 g/mL. How many molecules are in a 60.0 mL piece?



Liquid Mercury has a density of 13.55 g/mol. Find the volume occupied by 1.806 x 10^27 atoms of G