Question 1
The average atomic mass of element is 84.66
calculation
Average atomic mass is of an element is the sum of masses of its isotope each multiplied by its natural abundance.
that is.
[(50.50 x 83.75) /100] + [( 30.25 x 84.45)/100] + [(,9.25 x 87.40)/100]
= 42.29 +25.55 +16.82= 84.66
Question 2
fill the table
symbol element protons neutrons Electrons mass No Atomic No
Cs-133 cesium 55 78 55 133 55
Na-23 sodium 11 12 11 23 11
Ne 22 Neon 10 12 1 0 22 10
Au -197 Gold 79 118 79 197 79
Question 3
The moles of tungsten that are in 93.50 grams are 0.509 moles
moles= mass/molar mass
molar mass of tungsten = 183.84 g/mol
moles is therefore= 93.50 g/ 183.84 g/mol= 0.509 moles
question 4
CO2 molecules at STP are 1.547 x10^23 molecules
Step 1: find the moles of CO2
At STP 1 mole of a gas= 22.4 l
? =5.75 L
by cross multiplication
= (5.75 L x 1 mole)/ 22.4 L= 0.257 moles
Step 2: use the Avogadro's constant to calculate the number of molecule
that is 1 moles = 6.02 x10^23 molecules
0.257 moles= ? molecules
by cross multiplication
=(0.257 moles x 6.02 x 10^23) / 1 mole = 1.547 x10^23 molecules
question 5
The number of calcium atoms is 3.82 x10^24 atoms
Step 1: find the moles of calcium
= 253.50 g /40 g/mol= 6.34 moles
Step 2: use the Avogadro's law to calculate the number of atoms.
that is 1 mole= 6.02 x 10^23 atoms
6.34 moles= ? atoms
=6.34 x 6.02 x10^23 = 3.82 x10^24 atoms
Question 6
mass in grams of NiBr2 is 126.97 grams
Step 1: by use of Avogadro's law constant calculate the number of moles
that is 1 mole = 6.02 x 10^23
? = 3.50 x 10^23
by cross multiplication
= (1 mole x 3.50 x10^23) / 6.02 x10^23 =0.581 moles
step 2: calculate the mass
mass =moles x molar mass
= 0.581 moles x218.53 g/mol = 126.97 grams
Bonus question
The molecules of water= 1.00 x10^26 molecules
calculation
convert liters into ml
= 3 x 1000 = 3000 ml
find the mass of water
= density x volume
=3000 x0.998 2994 grams
find the moles of water=mass/molar mass
= 2994 g/ 18 g/mol= 166.33 moles
Use Avogadro's constant to calculate the molecules of water
that is 1 mole= 6.02 x10^23 molecules
166.33 moles= ? molecules
= (166.33 moles x6.02 x10^23 molecules) / 1 mole = 1.00 x10^26 molecules
