[Dudealadude]

Hello, the question I am given is:
Determine the pH of the solution that is created by mixing 45.2 mL of 2.51×10-3 M HI with 154 mL of 9.32×10-4 M MgH2.
I'm pretty sure I have to make a net ionic equation, however I'm a bit confused about how to do that with these two chemicals and also how the coefficients work when calculating.
2HI + MgH2 + 4H2O = MgI2 + 4 H3O
This equation makes the most sense to me, although it doesn't seem right.

[Borek]

Are you sure it is MgH₂?

Your reaction equation is not balanced. H₃O⁺ is charged, that means you have +4 charge on the right but no charge on the left.

[rabolisk]

2HI + MgH2 + 4H2O = MgI2 + 4 H3O
This equation makes the most sense to me, although it doesn't seem right.

That equation doesn't seem right to me either. Imagine you were given HI and MgCO₃ instead. How would you do the reaction? Don't be thrown off by the hydride.

Although I agree with Borek that hydride is an unusual choice, considering that hydrides aren't really stable in aqueous solutions.

[AWK]

This is a problem with an excess of HI. As a good aproximation - imply strong acid in water after neutralization.