Chapter 4 Notes.pptx

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Chemistry:

Chapter 4

Three-step Problem Solving

Approach

1.

Analyze

– determine how you will find

the solution

2.

Calculate

– perform the calculation, this

may involve measurements

3.

Evaluate

– does the answer make sense,

Ex. What is the mass, in

grams, of a piece of lead that

has a volume of 19.84 cm

3

?

1. Analyze: list the knowns and the unknown.

Volume = 19.84 cm3 Density = mass/ volume

Density = 11.4 g/cm3 Mass = ?

2. Calculate: solve for the unknown. D = m / V so… m = D x V

Mass = 19.84 cm3 x 11.4 g/cm3 = 226 g

3. Evaluate: does the result make sense?

Practice: Solve the

following using correct

significant figures.

7.823 x 15.76 =

89

2

+ 17

3

+ 5

6

=

123.3

Practice What is the mass,

in grams of a piece of lead

that has a volume of 8.73

cm

3

?



Knowns

o

V = 8.73 cm

o

D = 11.4 g/cm3

m = VD



Unknowns

o

m = ?



To solve:

o

m = (8.73 cm

)(11.4 g/cm

)

Practice: What is the volume, in cm3, of a sample of

cough syrup that has a mass of 20.0g and a density of 11.4 g/cm3?



Knowns

o

M = 20.0 g

o

D = 11.4 g/cm

o

V = M / D



Unknowns

o

V = ?



To Solve

o

V = 20.0 g

o

11.4 g/cm

o

V =

Dimensional Analysis



To convert from one unit to another,

we will use a problem solving method

called

dimensional analysis

.



This method uses equalities or

conversion factors

to change one

unit into another.

o

Example: If someone gives you 32

How many inches are there in 4 ft?

 48 inches

 What did you have to know in order to figure that

out?

o _{1 ft = 12 inches}

 1 ft = 12 inches is a conversion factor. It can

be written as a fraction where the numerator and the denominator are equivalent but have different units. For example, we can use the following

conversion factors for changing between inches and feet:

Some Handy

Conversions

 Let’s look at a meter stick and list all the

conversions we can get from it.

 We can say that one meter is equal to…

o _{10 dm} o _{100 cm} o _{1000 mm}

 Now let’s use those prefixes to figure out how we

can modify units for liters.

Practicing with

Dimensional Analysis

 63 cm = ____ mm

 2.5 L = _____ mL

 1 m2 = _____ cm2

Practice: How many atoms are

in 7.00g of gold?

(1 atom of gold = 3.271 x 10

g)

7.00 g

gold

3.271 x 10

g

gold

1 atom

gold

= 2.14 x 10

Bellwork, Thursday,

September 13, 2012



Complete the following conversion:

(Show you work using dimensional

analysis)

•

45.00 m = _____________ mm

•

5.34 x 10

L = ______________ mL

Wednesday’s

Homework

1. Write at least five conversion factors using the

following equalities.

1000 mg = 1 g = 1 ml water = 0.001 L water

2. Use a meter stick to fill in the following:

_1_meter = ______ cm = _______ km = ______mm

3. A filled flask that can hold 158.238 g of water at

4°C can hold only 127.339 g of ethanol. What is the density of ethanol at 4°C? (hint: water’s density is 1.0 g/mL. You may need to use a proportion!)

4. If 100 cm = 1 m, how many cm3 are in 1 m3?

5. Find the mass of a 5.3 mL sample of lead in

Some dimensional analysis

problems require several steps 

How many seconds are in 5.0

days?

 5.0 days 24 hours60 min 60sec = 430,000 sec  1 day 1 hour 1 min

How many students are in a 10

room building if each

classroom contains 25

students?

10 rooms 25 students = 250

Students

An example of this would be the conversion of

speed in

miles per hour

to

meters per second

. 

An object was traveling at 400. m/min. What was

its speed in cm/s?



400. m 100 cm 1 min = 667 cm/sec



1 min 1 m 60sec

Convert 423 m/sec to km/min.

423 m 1 km 60sec = 25.4

km/min

The density of manganese is

7.21 g/cm

. What is the

density of manganese

expressed in units of kg/m

?



7.21 g 1 kg 100

cm

= 7210 kg/m

Bellwork: Friday,

September 15



A cheetah can run 112 km/h over a

100 m distance. What is this speed

in meters per second?



The density of dry air measured at

25

C is 1.19 x 10

g/cm

. What is the

Thursday Homework

1. Which gas is cheaper based on the sales prices below?

(show your calculations!)

1 L = 0.261 gal Gas A: $2.53/gallon Gas B:$0.45/ liter

2. Set up the equalities for the story and solve.

Marie Currie wanted to research an unknown

radioactive element (Sneezium Sz) that cost $534 per gram. Sneezium has a density of 55.67g/cm3, if Marie

needs 0.35 L of Sneezium, how much money should

Bellwork: Monday,

September 17



Earth is approximately 1.5 x 10

km

from the sun. How many minutes

does it take light to travel from the

sun to Earth? The speed of light is

3.0 x 10

m/s.



Convert 7.86 g/cm

to milligrams per

Bellwork: Tuesday,

Septmeber 18



The density of water is 1.0g/mL.

How many deciliters will fill a 0.5 L

bottle?



Fill in the following table:

mg

g

cg

kg

28.3

6.6 x

10

Monday’s Homework

1. Fill-in the following equalities

2. Anna wants to buy a one carat diamond. One carat

equals 200 mg. If a diamond is 0.600 carat, what is the mass of the diamond in ounces? 1 kg = 2.205 pounds 1 pound = 16 ounces

3. The speed of a rocket is measured and found to be 145.3

m/sec. What is the rocket’s speed in km/hr?

4. The moon is 250,000 miles away. How many feet is it

from earth?

_________ m = 1 km

_________ cL = 1 L

_________ mg = 1 g

_______ sec = 1 min _______ cm3 _{= 1 ml}