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Question

Which of the following correctly describes how to make 2.9 M solutiion of aqueous HF ?

1) Obtain 2.90g of HF crystals , add to volumetric flask and then dilute to the 1.0 L line.

2) Obtain 5.8 g of HF crystals,add to a votumetric flask,then dilute to 100 mL line.

3) Obtain 5.8 g of HF crystals, measure 100 mL in volumetric flask, then pour the crystals into the volumetric flask.

4) Obtain 58.029 of NaOH crystals, measure 1.0L in a volumetric flask, the pour the crystals into the volumetric flask.

Answer 1

The definition of M (molarity) is moles of solute per liter of solution.

2.9 M HF means 2.9 moles of HF in 1 liter of solution

Molar mass HF = 1 + 19 = 20 g/mol

To make 2.9 mols/liter we need 2.9 moles x 20 g/mol = 58 g in 1 liter of solution

This wold be the same as 5.8 g in 100 ml of solution

So Answer (2) is correct