Hi everybody. .

I have also another 2 Q. . I hope that I get help. .



1- A 0.8720g sample of a mixture cinsisting solely of sodium brpmide and potassium bromide yields 1.505g of silver bromide .
What are the percentages of the two salts in the sample?





2- Nickel dimethylglyoxime (H2C4H6O2N2) is a bulky precipitate that is inconvenient to manipulate in amounts greater than 175 mg . The amount of nickel in a type of permanent - magnet alloy ranges between 24 and 35% . Calculate the simple size that should not be exceeded when analyzing these alloys for nickel.

1. [First time I see such a question :o]

Find the amount of moles of silver bromide you have. I will denote it by 'z'.

You have the masses of NaBr and KBr which are 102.9 and 119 respectively.
Let x be the amount of moles of NaBr and y that of KBr.
You will therefore have x moles of Br^- together with y moles of Br^-.

Therefore you have :


And using the masses;
[math]102.9x + 119y = 0.8720[math]

Using simultaneous equations, solve for x and y, the amount of moles of each compound. From there, find the mass of that amount, and the percentage mass will be automatic.

2. [That too, a new one to me :eek:]

Ok, find the amount of moles of the bulky precipitate from 175 mg. Then, find the number of moles of nickel present in that precipitate.

Find the mass of that amount of moles.

That mass makes up to at most 35% of the magnet. Find the mass of 100% using proportions. That's your answer, the maximum weight of the sample to be tested.

Now, if the amount was a minimum, you'll see that the precipitate will not exceed the 175 mg, since there are less nickel.

I hope it helped! :)

Quote:

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Originally Posted by Unknown008

And using the masses;
[math]102.9x + 119y = 0.8720[math]

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from where you get 0.8720 ?

Quote:

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Originally Posted by Unknown008

That mass makes up to at most 35% of the magnet. Find the mass of 100% using proportions. That's your answer, the maximum weight of the sample to be tested.

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I didn't understand this point. .

Quote:

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Originally Posted by Asoom

from where you get 0.8720 ?

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Let's work backwards. How would you calculate the total mass of NaBr and KBr together?

You take the number of moles of the NaBr and KBr, multiply each one by it's corresponding relative mass and add them up. It turns out that the total is 0.8720 g as given in the question.

I said let x be the number of moles of NaBr, so, the mass due to NaBr is (23+79.9) x and that due to KBr is (39.1+79.9) . Adding them up should give the given sample mass; 0.8720 g

Quote:

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Originally Posted by Asoom

I didnt understand this point . .

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This is proportions. You know the mass, or 'value' of 35%, find that of 100%.

It's like:
25 % the mass of this apple is 1.25 g.
100 % (total) mass of the apple is (1.25/25 * 100) = 5 g

Here, your sample has a certain mass you don't know. It's at 100% mass.
You know that 35 % of that mass is nickel.

35% has a mass that you find, you can find the 100%.

Is it clear enough? Or still... sort of confused?

I don't no heheeh