# Homework Help: Two Tricky Problems

1. Mar 7, 2006

### Raza

A Tricky Problem

1. If 4.74g of magnesium is reacted with 4.000L of 2.500mol/L hydrochloric acid, calculate the change in temperature for the hydrochloric acid.
I think the equation is Mg(s) + HCl(l) ------> MgCl(2)(l) + H(2)(g)

I know these two equations will be in use.
1. Q=mTc
2. H=Q/n

I suppose I find the mol of magnesium.
4.74g x 1mol/24.305g
=0.195mol

Thank You.

Last edited: Mar 7, 2006
2. Mar 8, 2006

### Emieno

The change in temporature in this case is caused by the change in mol left after and used during the reaction.
From the ratio 1:2:1:1 you calculate the HCl left. mol_left=given_mol - used_mol, then find the grams of HCl used.
Use the table given in the textbook you are reading to check for c, reread the chapter to understand Q and also note the starting temperature that you need to compare with I think already stated in the problem itself. dQ=mcdT

3. Mar 9, 2006

### GCT

In addition to what Emieno said, you should have been given the relevant enthalpy of reaction (or simply standard change in the energy of the reaction).