[Tony Ramirez]

Unfortunately I've forgotten my high school chemistry, however it was many years ago!

I've used a commercial human safe borax formulation (Ant Rid) in the past to kill ants, however the product is not available in the country where I'm currently living. You add borax to a mixture of water, sugar and/or honey. The ants eat the bait and after a few days they die.

For the bait to be most effective, it needs to be formulated at 11.83 g/L Boron. The chemical formula of the borax powder, that I have is Na2B4O7.5H20 . If I did my math correctly then the molecular weight of this is 291.32.

Could someone kindly remind me how to calculate the amount of borax I need to add to make up (let's say 100 ml) at the correct concentration. Thanks

[Borek]

For the bait to be most effective, it needs to be formulated at 11.83 g/L Boron. The chemical formula of the borax powder, that I have is Na2B4O7.5H20

OK

If I did my math correctly then the molecular weight of this is 291.32.

Looks OK. I would add g/mol as a unit to make sure we talk about the same thing, lack of units is the most common source of errors.

Let's say you took 1/4 mole of borax by weighting exactly 291.32/4=72.8 grams of the substance. How many of these 72.8 grams would be boron? If you dissolve all this in some water and fill up to 1 L (this way you can be sure what the exact volume is, if you add the solid to 1 L of water you will end with some more solution than you planned), how much boron would you have per 1 L of solution?

Does it push you in the right direction?

[billnotgatez]

Just FYI

From WIKI

Borax
https://en.wikipedia.org/wiki/Borax

Chemical formula
Na2B4O7·10H2O
 or
Na2[B4O5(OH)4]·8H2O

Molar mass
201.22 (anhydrous)
381.38 (decahydrate)

Appearance
white solid

Density
2.4 g/cm3 (anhydrous, solid)
1.73 g/cm3 (decahydrate, solid)


The term borax is often used for a number of closely related minerals or chemical compounds that differ in their crystal water content:

anhydrous sodium tetraborate, Na2B4O7
sodium tetraborate pentahydrate, Na2B4O7·5H2O
sodium tetraborate decahydrate, Na2B4O7·10H2O or equivalently the octahydrate, Na2B4O5(OH)4·8H2O
From the chemical perspective, borax contains the [B4O5(OH)4]2− ion. In this structure, there are two four-coordinate boron centers and two three-coordinate boron centers.

edit
I do not know why WIKI did not include the pentahydrate in its list of values
other sources list close to 291.32 g/mol for molecular weight for pentahydrate

[Tony Ramirez]

Looks OK. I would add g/mol as a unit to make sure we talk about the same thing, lack of units is the most common source of errors.

Let's say you took 1/4 mole of borax by weighting exactly 291.32/4=72.8 grams of the substance. How many of these 72.8 grams would be boron? If you dissolve all this in some water and fill up to 1 L (this way you can be sure what the exact volume is, if you add the solid to 1 L of water you will end with some more solution than you planned), how much boron would you have per 1 L of solution?

Does it push you in the right direction?

Thanks for the reply.

So if I dissolved 1 mole or 291.32 g of the pentahydrate to make 1 litre, I should have 43.24 g of boron, since MW of boron is 10.81 and we have 4 boron atoms. To get the desired conc. of boron (11.83 g/L), I need to divide by 3.65. So if my reasoning is correct, the amount of pentahydrate to dissolve in 1 litre is 291.32/3.65 = 79.69 g.

[Borek]

Yep, 80 g/L looks OK (I doubt you will be able to weigh exactly 79.69 g, besides, exact composition of hydrates often depends on the humidity, so you can't be really sure it is exactly Na₂B₄O₇·5H₂O, amount of water can differ).