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Example Question - 1

The mass concentration in gdm-3 is mass ÷ volume in dm3
To convert the volume from cm3 to dm3 you divide by 1000.
A student heats 12.41g of hydrated sodium thiosulfate Na2S2O3.5H2O to remove the water of crystallisation.A white powder called sodium thiosulfate forms.Calculate the expected mass of sodium thiosulfate that forms.moles of Na2S2O3.5H2O = 12.41 ÷ 248.2 = 0.05molmoles of anhydrous Na2S2O3 = 0.05molmass of anhydrous Na2S2O3 = 0.05mol × 158.2= 7.91g

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Specification: AQA Chemistry (7404), AS-level, 3.1.2.5 Balanced equations and associated calculations and 3.1.2.2 The mole and the Avogadro constant. Topic: Amount of Substance.

Key terms in this lesson

Mole
The amount of substance that contains as many particles as there are atoms in exactly 12g of carbon-12.
Mass concentration
The mass of solute, in grams, dissolved in each dm3 of solution.

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