# Chem 11 Section Assignment 3.1

Section Assignment 3.1: Determining Atomic Mass

Instructions

Perform the following calculations. Take care to give answers with the appropriate units and significant figures. Show your solution method clearly. The use of a Periodic Table will be required. 1. Element "E" (not a real symbol for an element), has three naturally occurring isotopes: 60% 228E, 25.0% 222E, and 15.0% 232E. Use this information to determine the atomic mass of element "E".

Atomic Mass = (0.60 x 228g/mol) + (0.25 x 222g/mol) + (0.15 x 232g/mol)

= 227.1g/mol (Actinium)

2. Calculate the approximate percent abundance of gallium's isotopes (69Ga, 71Ga) in a sample of gallium metal.

Ga atomic mass is 69.72g/mol. Let x be the % of 69Ga as a decimal and therefore 1.000 – x is the % of 71Ga as a decimal.

(x)69 + (1.00 – x)71 = 69.72

69x + (71.00 – 71x) = 69.72

71.00 – 2x = 69.72

71.00 – 69.72 = 1.28 = 2x

x = 1.28/2 = 0.64 or 64%, 69Ga = 64%

1.00 – x = 1.00 – 0.64 = 0.36 or 36%, 71Ga = 36%

3. Vanadium, atomic number 23, has two isotopes, one of which is 51V. If the percent of 51V is 94, then what must the mass number of the other isotope be?

51V is 94% & xV is 6%, V atomic mass = 50.94g/mol

Let x be the mass # of the other isotope

% of the other isotope = 100 – 94 = 6%

(51 x 0.94) + (X x 0.06) = 50.94

47.94 + 0.06x = 50.94

0.06x = 50.94 – 47.94 = 3.00

x = 3.00/0.06 = 50

Isotope mass # = 50V

4. Convert the following masses into moles.

A. 135 grams of Aluminum.

Molar Mass of Aluminum = 26.98g/mol

# moles Al = 135g x 1 mole = 5.0 moles

26.98g

B. 1.0 grams of Copper.

Copper Molar Mass = 63.55g/mol

# moles Cu = 1.0g x 1 mole = 0.016 moles

63.55g

5. Convert the following moles into masses.

A. 0.160 moles of Magnesium.

Mg Molar Mass = 24.31g/mol...

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