Calculate the number of O atoms in 0.364 g of CaSO4 · 2H2O

= 7.66 x 10²¹  oxygen atoms in 0.364 grams of  CaSO₄·2H₂O

Explanation:

For problems like this posting, one needs an understanding of the following topics:

The definition of the mole

1 mole of substance = mass in grams of substance containing 1 Avogadro's Number ( = 6.023 x 10²³ ) of particles of the specified substance. This is generally one formula weight of the substance of interest. From this, the following equivalent relationships should be memorized:

  1 mole = 1 formula weight = 1 mole weight (g)= 6.023 x 10²³ particles

Converting grams to moles:

Given grams => moles = grams/gram formula wt

Converting moles to grams:

Given moles => grams = moles x gram formula wt

Calculate the number of O atoms in 0.364 g of CaSO₄ · 2H₂O.

Given mass CaSO₄ · 2H₂O = 0.364 grams

Formula Wt CaSO₄ · 2H₂O = 172 g/mole

moles CaSO₄ · 2H₂O = mass CaSO4 · 2H2O / formula Wt. CaSO₄ · 2H₂O

= 0.364 g CaSO₄·2H₂O / 172 g CaSO4·2H2O

= (0.364/172) mole CaSO₄·2H₂O

= 2.12 x 10⁻³ mole CaSO₄·2H₂O    

∴ number of Oxy (O) atoms in 0.364 grams CaSO₄·2H₂O

=  (2.12 x 10⁻³ mole CaSO₄ · 2H₂O)(6.023 x 10²³ molecules CaSO₄· 2H₂O/ mole)

= 1.276876 x 10²¹molecules CaSO₄· 2H₂O  CaSO₄2H₂O

= 1.276876 x 10²¹ molecules CaSO₄· 2H₂O   x   6 oxygen atoms / molecule

= 7.661256 x 10²¹  oxygen atoms in 0.364 grams of  CaSO₄·2H₂O

= 7.66 x 10²¹  oxygen atoms in 0.364 grams of  CaSO₄·2H₂O

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