Question

Write the correct formula for the substance having a mass of 322 g and composed of 63.73 % Na2S2O3 and 36.27 % water.The lab is called The Quantitative Determination of an Empirical Formula.

Answer 1

Calculation of the empirical formula

To determine the empirical formula or minimum formula, we must follow these two steps:

1) Calculate the number of moles of atoms of each element.

2) Divide the result by the smallest value found.

First, let's find the mass of N₂S₂O₃:

100% ---- 322 g

63.73% ---- x

x = 205.2106 g of N₂S₂O₃

Now let's find the quantity of in moles:

n = m/M

where:

n = quantify in moles

m = mass in grams

M = molar mass in g/mol

So:

m = 205.2106 g of N₂S₂O₃

M = 158 g/mol

Let's replace the values in the equation:

n = 205.2106/158

n = 1.30 moles g of N₂S₂O₃

Let's do the same calculation for water (H₂O):

First, let's find the mass of H₂O:

100% ---- 322 g

36.27% ---- x

x = 116.8 g of H₂O

Now let's find the quantity of in moles:

n = m/M (M of water is 18 g/mol)

n = 116.8/18

n = 6.5 moles of H₂O

Now 2) Divide the result by the smallest value found:

6.5 moles of H₂O and 1.30 moles g of N₂S₂O₃

The smallest value found was 1.30

6.5/1.3 = 5 of H₂O

1.3/1.3 = 1 of N₂S₂O₃

So the empirical formula is:

N₂S₂O₃.5H₂O