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Water of crystallisation in washing soda

Titration to find water of crystallisation; errors and validity

Washing soda is

\ce{Na2CO3.xH2O}

. 1.792 g of crystals was dissolved and made up to 250.0 mL in a volumetric flask. 20.00 mL aliquots were titrated with 0.0500 mol L

^{-1}

HCl using methyl orange. Titres (mL): 20.55, 20.05, 20.00, 20.10.

\ce{Na2CO3 + 2HCl -> 2NaCl + H2O + CO2}

. M (g mol

^{-1}

): Na 23.0, C 12.0, O 16.0, H 1.0.

Facts you may need

n = cV
mole ratios
aliquot scaling
concordant titres

(a)

Choose the titres to use, explaining your choice, and calculate the average titre.

(b)

Calculate the value of x.

(c)

Explain why methyl orange is used rather than phenolphthalein for this titration.

(d)

The crystals had been left in a warm room and had lost some water of crystallisation (efflorescence) before weighing. Explain the effect on the calculated value of x.