Chem(qualitative analysis)

Today, I have done a qualitative analysis to deduce what the sample is. (It is very strange =.=)
I was given a solid sample.
And I had to do a series of tests.
1. Appearance: It exists as a white powder.
2. Flame test: It gives a golden yellow flame.
3. Solubility in water: It is solube in water.
4. pH: Around 11 - 12
5. Action of 2M HCl on an aqueous solution of the sample: No observable change
6. Action of 1M H2SO4 on an aqueous solution of the sample: No observable change
7. Action of 2M NaOH on an aqueous solution of the sample: No observable change
8. Action of aqueous NH3 on an aqueous solution of the sample: No observable change
9. Action of a few drops of 0.02M acidified KMnO4: The solution turns to purple and a little gas bubbles evolved.
10. Action of aqueous silver nitrate: The solution turns to yellowish brown.
11. Action of aqueous barium chloride: White precipitate is formed.

From 2, it is obvious that Na+ is present in the sample.
From 11, it is obvious that SO4- is already presented in the sample.
So I deduced it the sample is Na2SO4.

BUT REFER TO test 3, the pH is around 11-12, which indicated a strong base is present in the sample. But is Na2SO4 a strong base?

I just wanna put this to share and discuss. Hope a guy can answer it.