SME
Grade 7 Science · Problem type
Mass before and after a reaction
Use this for “20 g of A and 10 g of B react completely. How much C forms?” and “The sealed bottle weighed 230 g; opened, 228.5 g. How much gas escaped?”
Diagram
230 g − 228.5 g = 1.5 g.
That is the gas that escaped.
This is an example. Type your own values to start a new problem.
a (g)
+
b (g)
=
s (g)
s (g)
−
r (g)
=
e (g)
Formulas
a
+
b
=
s
(mass
of
the
first
substance
+
mass
of
the
second
substance
=
total
mass,
sealed)
4 + 226 = 230
s
−
r
=
e
(total
mass,
sealed
−
mass
after
opening
=
mass
of
gas
that
escaped)
230 − 228.5 = 1.5
Assumptions
•
In a closed container the mass does not change in a reaction: the atoms are all still there.
•
If a gas is made and escapes, the scale reads less by exactly the mass of the gas.
•
20 g and 10 g reacting completely make 30 g of product, no more and no less.
Step-by-step
WE KNOW
a = 4 g (mass of the first substance), b = 226 g (mass of the second substance), r = 228.5 g (mass after opening)
FIND
total mass, sealed (s), mass of gas that escaped (e)
Step 1 · Find total mass, sealed (s)
Use a + b = s (mass of the first substance + mass of the second substance = total mass, sealed)
Sealed, everything that went in is still on the scale: add the two masses.
s
=
4
+
226
s
=
230
g
Step 2 · Find mass of gas that escaped (e)
Use s − r = e (total mass, sealed − mass after opening = mass of gas that escaped)
The scale reads less after opening by exactly the gas that left.
e
=
230
−
228.5
e
=
1.5
g
Check
Put the numbers back in. Both sides should match.
4 + 226 = 230 ✓
230 − 228.5 = 1.5 ✓
Topic check
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