[grzegorzzewski]

There is a following notation in described experiment:

To a vigorously stirred solution of 1.5 g of tetraoctylammonium bromide (2.5 equiv) in 80 mL of toluene was added 0.31 g of HAuCl4 * xH2O (1 equiv) in 25 mL of deionized water. The yellow HAuCl4 * xH2O aqueous solution quickly cleared and the toluene phase became orange-brown as the AuCl4- was transferred into it. The organic phase was isolated, the desired amount of dodecanethiol was added, and the resulting solution was stirred for 10 min at room temperature. (For dodecanethiol:AuCl4- >=2, the orange-brown solution became either very pale yellow or colorless within 5 min.) Adjusting the solution temperature as desired for the reduction step, the reaction solution was then vigorously stirred and NaBH4 (0.38 g, 10 equiv) in 25 mL of deionized water was added over periods of either 10 s, 2 min, or 15 min (referred to as fd, md, and sd for fast, medium, and slow delivery, respectively).

What does mean notation of HAuCl4 * xH2O? Can someone explain please?
The source is here http://www.chemistry.sjsu.edu/rterrill/ped/146/reader/la970588w_mpc_prep.pdf

[Borek]

Hydrate of not well known composition. However, somehow they determined that 0.31 g of the solid contains exactly the required amount of HAuCl₄.

[grzegorzzewski]

So I understand that in this experiment, were mixed 0.31 g of HAuCl4 with a determinate amount of H2O (e.g. such that the molar ratio was 2:1), and then this mixture was added to 25 ml of deionized water.
How can I determine the amount of water, if I wanted to repeat the experiment?
Sorry for the maybe trivial question, but I'm not a chemist and this gives me some difficulties.

[Arkcon]

No.  Some crystalline solids have water associated with the crystal structure, even if nominally dry.  The x H2O is not added water, its just there.  Compare with this common reagent -- copper sulfate: http://en.wikipedia.org/wiki/Copper%28II%29_sulfate

[grzegorzzewski]

All right. This is the final compound. I understand you.

Another problem is the determine of molar ratio. There is the following description:
"Silver nano particles were formed by adding NaBH4(0.5 g) dissolved in 2-
propanol (85 mL) to a solution of AgNO3 (0.5 g) and alkane thiol in 2-propanol (85 mL).
The size of the nano-particles was controlled by the ratio of alkanethiol to AgNO3"

If the mole ratio (MR) of alkanethiol to AgNO3 is 2:1, I calculate amount of butanethiol (C4H10S) according to the following formula:
m_butanethiol=MR*m_AgNO3; where m_x is the mass of compound x in grams.

It is O.K.?