# Tutor profile: Xiaoxiao C.

## Questions

### Subject: Chemistry

Balancing the following chemical equation: H2SO4 + Ca(OH)2 --> CaSO4 + H2O

First, we have to understand why we need to balance equations. We have to follow the Law of Conservation of mass, where in a closed system, the mass cannot be created nor destroyed. Apply to the balancing equations, we should have an equal number of each type of elements for both reactants and products. We can start counting how many of each atom are from both sides. On the reactant side, we have H=2 SO4=1 (instead of counting S=1 and O=4, we count SO4 as a whole, this is a polyatomic ion, and known as sulfate) Ca=1 OH=2 (another polyatomic ion, hydroxide) On the product side, first, we want to rewrite H2O as HOH, this way, it is easier to balance. H=1 SO4=1 Ca=1 OH=1 Compare the reactant and product side, we found that Hydrogen and hydroxide are not balanced (in other words, they do not agree with each other one the reactant and product side). We will need 2 H and 2 OH from the product side to balance out the H and OH from the reactant side. For the equation, we are going to put a 2 in front of H2O because this way, we will have 2 H and 2OH on the product side of the equation. The number in front of the chemical formula is known as a coefficient, we can only balance the chemical equations with coefficients. The final answer is H2SO4 + Ca(OH)2 --> CaSO4 +2 H2O Special note, when we have multiple polyatomic ions, we will need to use parentheses in the chemical formula, for example, Calcium hydroxide, Ca is +2 charge and OH is -1, we will need 2 OH to balance the charge, it becomes Ca(OH)2.

### Subject: Basic Chemistry

Write the chemical formula for sodium chloride.

Sodium chloride is an ionic compound, it is made from sodium atoms and chlorine atoms. We can find the chemical formula for both of them from the periodic table. Sodium is atomic number 11 from the periodic table. It locates at group 1, where only has one valence electron. Chlorine is atomic number 35 from the periodic table, it is from group 17 and it has 7 valence electrons. When the sodium and chlorine become compounds, they have to form bonds between sodium and chlorine, where sodium transfer its valence electron to the chlorine atom. Now sodium loses one electron (electrons are negative charges, each one of them is a -1 charge) and becomes positive 1 charge. Chlorine gains one electron and becomes a negative 1 charge. Elements become stable when they have 8 valence electrons, and now both sodium and chlorine have 8 valence electron, they only need one sodium to bond with chlorine and it will be satisfied ( 8 valence electrons for both elements), thus the chemical formula for sodium chloride is NaCl.

### Subject: Algebra

Solve the equation 5(- 3x - 2) - (x - 3) = -4(4x + 5) + 13

First, we need to simplify the equation. On the left-hand side, 5(-3x-2)-(x-3), 5 can be distributed with both -3x and 2, thus 5(-3x-2) =-15x-10. Then we can combine the variables inside the first parentheses with the second parentheses and become -15x-10-(x-3), because negative and negative become a positive, thus -15x-10-(x-3) = -15x-10-x+3, if we combine the variables with variable and number with numbers -15x-x-10+3=-16x-7 and this is the simplest form for left-hand side. We move on to the right-hand side, we are going to do the same thing with the number and variables inside the parentheses, -4(4x+5) = -16x-20. When we combine that with the leftover 13, it becomes -16x-20+13=-16x-7. Finally, we rewrite the equation 16x-7 = -16x-7. We are going to move all the variables to one side by adding 16x to both side 16x-7+16x = -16x -7 +16x, 32x-7=-7. And we are going to move all the number to one side by adding 7 to both sides of the equation 32x-7+7=-7+7, it becomes 32x=0 and x=0 is the final answer to the equation.

## Contact tutor

needs and Xiaoxiao will reply soon.