In a closed system, the total mass of the reactants compared with the total mass of the products is:
exactly equal
always greater
always smaller
impossible to predict
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WhyThe law of conservation of mass states that no atoms are lost or made in a chemical reaction, so the total mass of reactants equals the total mass of products in a closed system.
2Multiple choice · Easy
What is the relative formula mass (Mr) of water, H2O? (Ar: H = 1, O = 16)
18
17
34
16
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WhyMr = (2 x 1) + 16 = 18. Add the relative atomic masses of all atoms shown in the formula.
3Multiple choice · Easy
Which unit is used to measure the concentration of a solution in this topic?
g/dm3
g/s
cm only
degrees C
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WhyConcentration is commonly expressed in grams per cubic decimetre, written g/dm3.
4Multiple choice · Easy
When magnesium is burned in air in an open container, the mass of the solid appears to increase. Why?
Oxygen from the air is added to the magnesium
Mass is created in the reaction
The magnesium absorbs heat which has mass
Water is produced and stays in the solid
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WhyOxygen gas from the air combines with the magnesium to form magnesium oxide; the added mass of oxygen makes the solid heavier.
5Multiple choice · Easy
What is the relative formula mass (Mr) of carbon dioxide, CO2? (Ar: C = 12, O = 16)
44
28
32
56
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WhyMr = 12 + (2 x 16) = 12 + 32 = 44.
6Fill in the blank · Easy
The law of conservation of states that no atoms are lost or made during a chemical reaction.
Answer:
mass
WhyMass is conserved because the same atoms are simply rearranged into products.
7Multiple choice · Medium
A reaction gives off a gas in an open flask, so the measured mass of the flask and contents falls during the reaction. What best explains this?
A gas product escapes from the open flask
Mass is destroyed in the reaction
The reactants gained oxygen
The balance is faulty by definition
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WhyA gaseous product escapes the open flask, so its mass is no longer counted; the total mass is still conserved if the escaped gas is included.
8Multiple choice · Medium
What is the relative formula mass (Mr) of calcium carbonate, CaCO3? (Ar: Ca = 40, C = 12, O = 16)
100
68
84
108
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WhyMr = 40 + 12 + (3 x 16) = 40 + 12 + 48 = 100.
9Multiple choice · Medium
What is the relative formula mass (Mr) of magnesium nitrate, Mg(NO3)2? (Ar: Mg = 24, N = 14, O = 16)
148
86
102
172
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WhyMr = 24 + 2 x (14 + 3 x 16) = 24 + 2 x 62 = 24 + 124 = 148.
10Multiple choice · Medium
6.0 g of carbon reacts completely with 16.0 g of oxygen to form carbon dioxide. What mass of carbon dioxide is produced?
22.0 g
10.0 g
16.0 g
96.0 g
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WhyBy conservation of mass, mass of products = mass of reactants = 6.0 + 16.0 = 22.0 g.
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