Answer:
18 is a
19 is c
20 i am not sure
Explanation:
18)nucleus contains protons and neutrons
Answer: 
Explanation: According to law of conservation of mass, mass can neither be created nor be destroyed.
Thus the mass of products has to be equal to the mass of reactants. The number of atoms of each element has to be same on reactant and product side. Thus chemical equations are balanced.
This is an example of combustion reaction in which hydrocarbon reacts with oxygen to form carbon dioxide and water. This is an exothermic reaction in which a large amount of heat is liberated, thus water also exists as vapors.
Given:
2.5 mol O2
Required:
Grams of water
Solution:
2H2 + O2 -> 2H2O
Moles of H2O = (2.5 mol O2)(2
moles H2O/1 mole O2) = 5 moles H2O
Molar mass of H2O = 18.02 g/mol
Mass of H2O = (5 moles H2O)(
18.02 g/mol)
<span>Mass of H2O =90.1g H2O</span>
The composite material is composed of carbon fiber and epoxy resins. Now, density is an intensive unit. So, to approach this problem, let's assume there is 1 gram of composite material. Thus, mass carbon + mass epoxy = 1 g.
Volume of composite material = 1 g / 1.615 g/cm³ = 0.619 cm³
Volume of carbon fibers = x g / 1.74 g/cm³
Volume of epoxy resin = (1 - x) g / 1.21 g/cm³
a.) V of composite = V of carbon fibers + V of epoxy resin
0.619 = x/1.74 + (1-x)/1.21
Solve for x,
x = 0.824 g carbon fibers
1-x = 0.176 g epoxy resins
Vol % of carbon fibers = [(0.824/1.74) ÷ 0.619]*100 =<em> 76.5%</em>
b.) Weight % of epoxy = 0.176 g epoxy/1 g composite * 100 = <em>17.6%</em>
Weight % of carbon fibers = 0.824 g carbon/1 g composite * 100 = <em>82.4%</em>
