Hello Duri Eun

The molecular weight of FeCl3.6H2O is 270.2.

So, add 0.0541g of Iron (III) chloride hexahydrate to 10ml ultrapure water.

You may weigh 0.0541g of Iron (III) chloride hexahydrate and transfer the Iron (III) chloride hexahydrate to a beaker of appropriate size. Add 5 mL of ultrapure water to the beaker to dissolve the salt. Transfer your solution to a 10 mL volumetric flask. Using ultrapure water, complete the solution's volume to reach 10 mL. You may prepare this solution on the day of the experiment.

It is better to prepare a fresh solution. This solution is sensitive to light.

Best.