# Chlorine Atomic Mass

• ››More information on molar mass and molecular weight. In chemistry, the formula weight is a quantity computed by multiplying the atomic weight (in atomic mass units) of each element in a chemical formula by the number of atoms of that element present in the formula, then adding all of these products together.
• Chlorine In 1961, the Commission recommended Ar (Cl) = 35.453 (1), based on the atomic weight of silver and on Ag/AgCl equivalent ratios determined chemically by many well-known authorities.

Chlorine-35 is composed of 17 protons, 18 neutrons, and 17 electrons. Chlorine-37 is composed of 17 protons, 20 neutrons, and 17 electrons. Chlorine-36 is composed of 17 protons, 19 neutrons, and 17 electrons. Trace amounts of radioactive 36 Cl exist in the environment, in a ratio of about 7×10 −13 to 1 with stable. ### Chlorine Atomic Mass Weighted

• Define atomic mass.
• Calculate atomic mass given relevant information about the isotopes.

### Have you ever tried to move a boulder?

You have a pile of rocks to move and need to decide what equipment you want to rent to move them. If the rocks are fairly small, you can get a shovel to pick them up. Larger rocks could be moved by hand, but big boulders will need some sort of mechanical scoop. The amount of each kind of rock will also determine how much time you will need to get the job done. Knowing the relative amounts of large, medium, and small rocks can be very useful in deciding how to approach the job.

Most elements occur naturally as a mixture of two or more isotopes. Table below shows the natural isotopes of several elements, along with the percent natural abundance of each.

 Element Isotope (symbol) Percent natural abundance Atomic mass (amu) Average atomic mass (amu) Hydrogen [latex]^1_1text{H}[/latex] 99.985 1.0078 1.0079 [latex]^2_1text{H}[/latex] 0.015 2.0141 [latex]^3_1text{H}[/latex] negligible 3.0160 Carbon [latex]^{12}_6text{C}[/latex] 98.89 12.000 12.011 [latex]^{13}_6text{C}[/latex] 1.11 13.003 [latex]^{14}_6text{C}[/latex] trace 14.003 Oxygen [latex]^{16}_8text{O}[/latex] 99.759 15.995 15.999 [latex]^{17}_8text{O}[/latex] 0.037 16.995 [latex]^{18}_8text{O}[/latex] 0.204 17.999 Chlorine [latex]^{35}_{17}text{Cl}[/latex] 75.77 34.969 35.453 [latex]^{37}_{17}text{Cl}[/latex] 24.23 36.966 Copper [latex]^{63}_{29}text{Cu}[/latex] 69.17 62.930 63.546 [latex]^{65}_{29}text{Cu}[/latex] 30.83 64.928

For some elements, one particular isotope predominates greatly over the other isotopes. Naturally occurring hydrogen is nearly all hydrogen-1 and naturally occurring oxygen is nearly all oxygen-16. For many other elements, however, more than one isotope may exist in more substantial quantities. Chlorine (atomic number 17) is a yellowish-green toxic gas. About three quarters of all chlorine atoms have 18 neutrons, giving those atoms a mass number of 35. About one quarter of all chlorine atoms have 20 neutrons, giving those atoms a mass number of 37. Were you to simply calculate the arithmetic average of the precise atomic masses, you would get 36.

[latex]displaystylefrac{left(34.969+36.966right)}{2}=35.968text{ amu}[/latex]

Clearly the actual average atomic mass from the last column of the table is significantly lower. Why? We need to take into account the percent natural abundances of each isotope in order to calculate what is called the weighted average. The atomic mass of an element is the weighted average of the atomic masses of the naturally occurring isotopes of that element. The sample problem below demonstrates how to calculate the atomic mass of chlorine.

Sample Problem: Calculating Atomic Mass

Use the atomic masses of each of the two isotopes of chlorine along with their percent abundances to calculate the average atomic mass of chlorine.

Step 1: List the known and unknown quantities and plan the problem.

Known

• chlorine-35: atomic mass = 34.969 amu and % abundance = 75.77%
• chlorine-37: atomic mass = 36.966 amu and % abundance = 24.23%

Unknown

• Average atomic mass of chlorine

Change each percent abundance into decimal form by dividing by 100. Multiply this value by the atomic mass of that isotope. Add together for each isotope to get the average atomic mass.

Step 2: Calculate

 chlorine-35 0.7577 × 34.969 = 26.50 amu chlorine-37 0.2423 × 26.966 = 8.957 amu average atomic mass 26.50 + 8.957 = 35.45 amu

Note: Applying significant figure rules results in the 35.45 amu result without excessive rounding error. In one step:

(0.7577 × 34.969) + (0.2423 × 36.966) = 35.45 amu

The calculated average atomic mass is closer to 35 than to 37 because a greater percentage of naturally occurring chlorine atoms have the mass number of 35. It agrees with the value from the Table above .

### Summary

• The atomic mass of an element is the weighted average of the atomic masses of the naturally occurring isotopes of that element.
• Calculations of atomic mass use the percent abundance of each isotope.

### Practice

Click on the link below to get some experience in atomic mass determinations:

### Review

1. Define atomic mass.
2. What information do you need to calculate atomic mass for an element?
3. Calculate the atomic mass for carbon using the data provided in Table below.
 mass number exact weight percent abundance Star wars™ - dark forces for mac download. 12 12.000000 98.90 13 13.003355 1.10

## Glossary • atomic mass: The weighted average of the atomic masses of the naturally occurring isotopes of that element.
• percent abundance: To calculate the weighted average, take into account the percent natural abundances of each isotope. The atomic mass of an element is the weighted average of the atomic masses of the naturally occurring isotopes of that element.
Show References

## References

1. Devon Fyson, based on image created by the U.S. Geological Survey. http://commons.wikimedia.org/wiki/File:Mass_Spectrometer_Schematic.svg.
2. George M. Groutas. http://www.flickr.com/photos/jorge-11/4777791888/.

#### Molar Mass, Molecular Weight and Elemental Composition Calculator

Molar mass of Cl is 35.4530 g/mol
Convert between Cl weight and moles
 Compound Moles Weight, g Cl

Elemental composition of Cl
ElementSymbolAtomic weightAtomsMass percent
ChlorineCl35.4531100.0000

 Mass percent composition Atomic percent composition

Sample reactions for Cl
 Equation Reaction type Na + Cl = NaCl synthesis H + Cl = HCl synthesis Fe + Cl = FeCl3 synthesis K + Cl = KCl synthesis KI + Cl = KCl + I single replacement

Formula in Hill system is Cl

### Computing molar mass (molar weight)

To calculate molar mass of a chemical compound enter its formula and click 'Compute'. In chemical formula you may use:
• Any chemical element. Capitalize the first letter in chemical symbol and use lower case for the remaining letters: Ca, Fe, Mg, Mn, S, O, H, C, N, Na, K, Cl, Al.
• Functional groups: D, Ph, Me, Et, Bu, AcAc, For, Ts, Tos, Bz, TMS, tBu, Bzl, Bn, Dmg
• parantesis () or brackets [].
• Common compound names.
Examples of molar mass computations: NaCl, Ca(OH)2, K4[Fe(CN)6], CuSO4*5H2O, water, nitric acid, potassium permanganate, ethanol, fructose.
Molar mass calculator also displays common compound name, Hill formula, elemental composition, mass percent composition, atomic percent compositions and allows to convert from weight to number of moles and vice versa.

### Computing molecular weight (molecular mass)

To calculate molecular weight of a chemical compound enter it's formula, specify its isotope mass number after each element in square brackets.
Examples of molecular weight computations: CO2, SO2.

### Definitions of molecular mass, molecular weight, molar mass and molar weight

• Molecular mass (molecular weight) is the mass of one molecule of a substance and is expressed in the unified atomic mass units (u). (1 u is equal to 1/12 the mass of one atom of carbon-12)
• Molar mass (molar weight) is the mass of one mole of a substance and is expressed in g/mol.
Weights of atoms and isotopes are from NIST article.
Related: Molecular weights of amino acids
molecular weights calculated today

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