CHEM 24112 Lecture Notes - Lecture 22: Magnesium, Unified Atomic Mass Unit, Weighted Arithmetic Mean

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CHEMISTRY 112 – DAY 21 Chapter 8 – Sections 1-3

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Chemical Composition

Objects do not need to have identical masses to be counted by weighing.

All we need to know is the average mass of the objects.

To count the atoms in a sample of a given element by weighing we must know the mass of the sample

and the average mass for that element.

Averaging the Mass of Similar Objects

Example: How many chocolate eggs are in the bag?

1. Not all chocolate eggs have the same mass.

2. Suppose we weigh 10 eggs and find:

3. Now we can find the average mass of an egg.

4. Finally we can divide to determine the # of eggs in the bag!

Exercise

A pile of marbles weigh 394.80 g. 10 marbles weigh 37.60 g. How many marbles are in the pile?

Atoms have very tiny masses so scientists created a unit to avoid using very small numbers.

1 atomic mass unit (amu) = 1.66 10–24 g

The average atomic mass for an element is the weighted average of the masses of all the isotopes of an

element.

Average Atomic Mass for Carbon

Even though natural carbon does not contain a single atom with mass 12.01, for our purposes, we can treat

carbon as though it is composed of only one type of atom with a mass of 12.01.

This enables us to count atoms of natural carbon by weighing a sample of carbon.

Example Using Atomic Mass Units

Calculate the mass (in amu) of 431 atoms of carbon.

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Document Summary

Objects do not need to have identical masses to be counted by weighing. All we need to know is the average mass of the objects. To count the atoms in a sample of a given element by weighing we must know the mass of the sample and the average mass for that element. Example: how many chocolate eggs are in the bag: not all chocolate eggs have the same mass, suppose we weigh 10 eggs and find, now we can find the average mass of an egg. Finally we can divide to determine the # of eggs in the bag! Atoms have very tiny masses so scientists created a unit to avoid using very small numbers. 1 atomic mass unit (amu) = 1. 66 10 24 g. The average atomic mass for an element is the weighted average of the masses of all the isotopes of an element.

Related Questions

Part A and B

I am doing the Atomic Mass of Metallo

You may have noticed on the periodic table that the atomic mass of an element is usually not a whole number. That happens because of isotopes. An atom that is missing a neutron or has an extra neutron is called an isotope. They are still the same element; however, they are just a little different from every other atom of the same element.

Most of the carbon atoms in the universe are Carbon-12, with 6 neutrons. A small percentage of carbon atoms are Carbon-13, with 7 neutrons, and an even smaller percentage are Carbon-14 and have 8 neutrons. Carbon-13 and Carbon-14 are isotopes of carbon.

Atomic mass is calculated by determining how many atoms of each isotope type are present in the universe. Since the isotopes of carbon make up only a small percentage of the carbon in the universe, the average of the masses of all carbon atoms is slightly higher than 12, so the atomic mass for carbon is actually 12.011.

Objective

Analyze the isotopes of âMetalloâ and to calculate its atomic mass

Materials

sample of metallo (bag of mixed nuts, bolts, and washers)

balance

Procedures

Obtain a sample of the new element âMetallo.â

Separate the three isotopes of Metallo (nuts, washers, bolts) and measure the mass of each isotope.

Count the numbers of each isotope.

Record all data in the table.

Data Table

	Nuts	Bolts	Washers	Totals
Total Mass (g)
Number

Average Mass (g)
Percent Abundance
Relative Abundance
Relative Mass

Analysis

Calculate the average mass of each isotope by dividing the total mass by the number of particles of that isotope. Record your answers on the Data Table.

Calculate the percent abundance of each isotope by dividing its number of particles by the total number of particles and multiplying by 100. Record your answers on the Data Table.

Calculate the relative abundance of each isotope by dividing the percent abundance from Step 2 by 100. Record your answers on the Data Table.

Calculate the relative mass of each isotope by multiplying its relative abundance from Step 3 by its average mass. Record your answers on the Data Table.

Calculate the average mass of all Metallo particles by adding the relative masses. This average mass is the atomic mass of Metallo.

Explain the difference between percent abundance and relative abundance. What is the result when you total the individual percent abundances? The individual relative abundances?

The percent abundance of each isotope tells you how many of each kind of isotope exist in every 100 particles. What does relative abundance tell you?

Compare the total values for Rows 3 and 6 in the Data Table. Explain why the totals differ and why the value in Row 6 best represents the atomic mass.

1. Which characteristic below describes ONLY a solid (not liquid or gas)?
a) definite shape b) particles in contact c) definite volume d) completely fills container
2. When iron rusts, it forms a reddish-brown solid. This is:
a) a physical change b) a chemical change c) no change at all
3. What is the name of FeSO4?
a) iron (I) sulfate b) iron (II) sulfate c) iron sulfite d) iron sulfide
4. A bottle label states that the bottle contains âdrinking water with minerals added for flavor. This would be an example of:
a) an element b) a compound c) a heterogeneous mixture d) a homogeneous mixture
5. Which answer has the correct number of significant figures for: length = 2.0 m + 0.2100 m?
a) 2.2 b) 2.21 c) 2.210 d) 2.2100
Place the answer with the correct number of significant figures on the lines provided below.
6. To drive one kilometer, how many centimeters should you drive?
_______________cm
7. A 10.0 gram block of gold has a density of 19.32 g/cm3. What is its volume?
______________cm3
8. What is the formula of sodium sulfide?
_________________________
9. What is the name of H2S?
_______________mm
10. How many neutrons are in 35Cl?
_______________ neutrons
For problems 11 through 13, show your work to receive full credit and use the correct number of significant figures in your final answer.
11. The density of nickel is 8.91 g/cm3. Calculate the mass of a nickel ball with a radius of 0.50 mm in grams.
(Ï = 3.1416; Vsphere = 4Ïr3/3)
12. The recommended adult dose of Elixophyllin , a drug used to treat asthma, is 6.00 mg/kg of body mass. Calculate the dose in milligrams for a 134-lb person.
13. It is 273 K outside. Do you need a jacket? Why or why not? (show your work for full credit)
14) Element X has three naturally occurring isotopes. The masses (amu) and % abundances of the isotopes are given in the table below. What is the average atomic mass of the element in amu?

isotope38 abundance of 5.07,mass of 37.919

isotope39x,abundance of 15.35,mass of 39.017

isotope 42,abundance of 79.85,mass of 42.111

15. Classify each of the following elements as alkali, alkaline earth, halogen or a noble gas:
a) He ________________________
b) F ________________________
c) Na ________________________
d) Ca ________________________
e) Br ________________________

CHEM 24112 Lecture Notes - Lecture 22: Magnesium, Unified Atomic Mass Unit, Weighted Arithmetic Mean

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