Question

A student wishes to prepare 125-mL of a 0.155 M manganese(II) bromide solution using solid manganese(II) bromide, a 125-mL volumetric flask, and deionized water. (a) How many grams of manganese(II) bromide must the student weigh out?

(b) Which of the following would NOT be an expected step in the procedure used by the student?

1)Add 125 mL of deionized water to the flask.

2)Carefully transfer the salt sample to the volumetric flask.

3)Carefully add water until the bottom of the meniscus sits on the mark on the neck of the flask.

Answer 1

First calculate the number of moles of MnBr₂

Number of moles = molarity * volume in L

= 0.155 * 125/1000

= 0.0194 Moles MnBr₂

Amount in g = number of mole s* molar mass

= 0.0194 Moles MnBr₂* 214.746 g/mol

= 4.161 g MnBr₂

(a)Thus 4.161 g MnBr₂ manganese(II) bromide must the student weigh out.

(b) the following would NOT be an expected step in the procedure used by the student:

1)Add 125 mL of deionized water to the flask. Because in doing so the volume of solution was greater than 125 ml

To make solution first add the weight amount in volumetric flask and then add water up to mark