Q unknown = -Q H2O
Q H2O= (100g x 4.18 J/g C x (25C - 20C)
Q H2O = 2090J
-2090J = (50g x c x (25C-90C)
-2090J = -3250c
.64 = c
I am a high school student and have done this problem a few months ago for my test, hope this helps
Answer: a) 
: Decomposition
b) 
: double displacement
c) 
: Synthesis (Combination)
d) 
: redox
Explanation:
Decomposition is a type of chemical reaction in which one reactant gives two or more than two products.
A double displacement reaction is one in which exchange of ions take place.
Synthesis reaction is a chemical reaction in which two reactants are combining to form one product.
Redox reaction is a type of chemical reaction in which oxidation and reduction takes place in one single reaction. The oxidation number of one element increases and the oxidation number of other element decreases.
The bottom of group 1. Francium (or Fr) is the element with the greatest metallic properties.
Francium is not a naturally-occurring element, however. It is man-made. There is an isotope of francium that exists naturally, but it's half life is so short that it decays almost instantly into a different element.
The naturally-occurring element with the highest metallic properties is cesium (or Cs), located right above francium.
Metallic characteristics decrease as you move from left to right on the periodic table.
Ionic molecules have higher boiling point than covalent molecules.
So that leaves Na2SO4 and NaCl.
The ionic molecule with higher charge will have higher boiling point.
Na2SO4 have ions with charge +-2.
NaCl have ions with charge +-1.
So the answer is C.
