How can I check if this mystery compound is strontium bromide?

Question

Trying to identify a mystery compound, here are some of my notes, I'm trying to get a second opinion on what it might be!

Observations

Appears as a white powder, some chunks are formed, sticking to side of tube slightly

Solubility

3 drops of distilled water, all of substance dissolved

Flame Test

Red Flame

Sodium Hydroxide Test

A white ppt was formed when mystery liquid was combined with sodium hydroxide

Concentrated sulfuric acid test

A yellow colored gas formed when 4 drops of sulfuric acid were added with substance in test tube

My thoughts based on these results narrow the cation down to lithium and strontium. After performing a precipitate test again with my two knowns (lithium, strontium)strontium was the only one to form a ppt. So I'm almost 100% sure that my cation is strontium.

I have the anion narrowed down to bromide, nitrate, and nitrite. It has to be one of those three! I'm leaning towards bromide as the anion, just interested on what you guys think from my notes.

Here are the options again, with my choice in bold:

Cation

• Lithium
• Strontium

Anion

• Bromide ?
• Nitrate
• Nitrite
• Iodine

To summarize:

I have a book with 16 possible cations, and 11 possible anions. Based on my results and the information given in the book, I've narrowed it down to 2 cations, lithium, and strontium, and 3 possibly 4 anions. I'm almost certain the cation is strontium, based on the fact that it formed the same white ppt. that the mystery substance did when mixed with sodium hydroxide. Lithium did not. As for the anion, I'm not sure, I'm leaning towards bromide, but was looking for a second opinion. I've been doing some research on how bromide reacts with sulfuric acid, and the results online have been almost identical to what happened with the unknown substance, however in my option book it says bromide and nitrate have very similar reactions.

If you have any more thoughts on how I could identify to anion, please let me know!

Answer 1

Relying on the fact that the compound is almost certainly a strontium compound, I'll look at the reactions with sulfuric acid of the specific anions:

• $\ce{SrBr2}$

$$\ce{SrBr2 + H2SO4 -> 2HBr + SrSO4}$$

Both compounds $\ce{HBr}$ and $\ce{SrSO4}$ are white or colourless. BUT! concentrated sulfuric acid oxidizes $\ce{HBr}$ to bromine gas:

$$\ce{2 HBr + H2SO4 → Br2 + SO2 + 2H2O}$$

And bromine gas has a yellow/red colour. So your compound is strontium bromide.

• $\ce{Sr(NO2)2}$

This is a yellow compound. As you mentioned, the compound was white, so it isn't strontium nitrite.

• $\ce{Sr(NO3)2}$

$$\ce{Sr(NO3)2 + H2SO4 -> 2HNO3 + SrSO4}$$

Both compounds $\ce{HNO3}$ and $\ce{SrSO4}$ are white or colorless. After looking some more stuff up, it turns out that sulfuric acid and nitric acid don't react (at room temperature) to produce any yellow/brown $\ce{N2O4}$ gas.