How to determine that how much solid of each sample do I need to add in water to

Question

How to determine that how much solid of each sample do I need to add in water to reach their molarity. 1) prepare 2.5 ml of 1 M kh2po4 in water 2) prepare 50 ml of 1.20 M of KI In water
In a 25-mL volumetric flask, use pipettes to introduce 2.5 mL of 0.040-M aniline + 2.5 ML of KH2PO4 +1.5 mL of 1.00 M NaOH. Add water to a total volume of about 20 mL and equilibrate this mixture and in a separate vessel the 0.040 M I2 in 1.20 M KI. After about 10 minutes, pipette 2.5 mL of the 0.040 M 12 in 1.20 M KI solution into the reaction vessel, dilute to the 25 mL mark, and mix well. Start the clock and transfer the reaction mixture to a spectrometer cell. Read the absorbance every 5 to 10 minutes for about 1 hour.

Explanation / Answer

1. prepare 2.5 ml of 1 M kh2po4 in water

For this calculation, we need the molar mass of KH2PO4, = 136.086 g/mol

By definition, 1 M, 1000 ml solution contains = 136.086 g of KH2PO4.

So, 1M, 1ml will contain = 136.086 /1000 g = 0.1361 g of KH2PO4

Or, 1M. 2.5 ml will contain = 2.5*0.1361 g = 0.340 g.

So, it can be prepared by dissolving 0.340 g to make 2.5 ml solution

or, a stock solution of 25 ml can be prepared to minimze the error by dissolving 3.4 g in 25 ml of solution and then pipette out 2.5 ml solution from there.

2. prepare 50 ml of 1.20 M of KI In water

Similar way, molar mass of KI = 166 g/mol

So, for 50 ml, 1.20M solution, KI = 50*1.20*166/1000 g = 9.96 g

Therefore, 9.96 g of KI to be dissolved to get 50 ml solution.

Or any volume of of stock solution can be prepared using the proportion amount.

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