A school science club investigates trends down Group I, the alkali metals, using samples of lithium and sodium. (a) A sample of lithium has a mass of 0.87 g and a volume of 1.63 cm^3. Calculate the density of this lithium sample, giving your answer to 2 significant figures. [2]…

Answer: ((a)) density = mass / volume = 0.87 / 1.63 = 0.53 g/cm3 (2 s.f.) ((b)) density = 1.45 / 1.50 = 0.97 g/cm3 (2 s.f.); density increases going down Group I (sodium's density, 0.97 g/cm3, is greater than lithium's, 0.53 g/cm3) ((c)(i)) 84 / 12 = 7; the sodium cube reacts 7 times faster than the lithium cube ((c)(ii)) Potassium would react faster than sodium, because reactivity increases going down Group I. Part (a): Density of the lithium sample Density is calculated using: density = mass/volume For the lithium sample: density = 0.87/1.63 = 0.5337… g/cm^3 Rounded to 2 significant figures, the density of lithium is 0.53 g/cm^3. Part (b): Dens…

IGCSE Chemistry 0620: Group I Density and Reactivity Trends (Calculations)

A school science club investigates trends down Group I, the alkali metals, using samples of lithium and sodium.

(a) A sample of lithium has a mass of $0.87\ \text{g}$ and a volume of $1.63\ \text{cm}^3$ . Calculate the density of this lithium sample, giving your answer to 2 significant figures. [2]

(b) A sample of sodium has a mass of $1.45\ \text{g}$ and a volume of $1.50\ \text{cm}^3$ . Calculate the density of this sodium sample, giving your answer to 2 significant figures, and state how density changes going down Group I based on your two results. [3]

(c) The club also times how long an identical-sized cube of each metal takes to react completely with excess water. A lithium cube takes 84 s to react completely, and a sodium cube takes 12 s to react completely.

(i) Calculate how many times faster the sodium cube reacts than the lithium cube. [2]

(ii) Predict whether a same-sized cube of potassium would react faster or slower than the sodium cube. Give a reason for your answer. [2]

Part (a): Density of the lithium sample

Density is calculated using: $\text{density} = \frac{\text{mass}}{\text{volume}}$

For the lithium sample: $\text{density} = \frac{0.87}{1.63} = 0.5337\ldots\ \text{g/cm}^3$

Rounded to 2 significant figures, the density of lithium is $\boxed{0.53}\ \text{g/cm}^3$ .

Part (b): Density of the sodium sample, and the trend

For the sodium sample: $\text{density} = \frac{1.45}{1.50} = 0.9667\ldots\ \text{g/cm}^3$

Rounded to 2 significant figures, the density of sodium is $\boxed{0.97}\ \text{g/cm}^3$ .

Comparing the two results: lithium has a density of $0.53\ \text{g/cm}^3$ and sodium has a density of $0.97\ \text{g/cm}^3$ . Since $0.97 > 0.53$ , density increases going down Group I.

Part (c)(i): Comparing reaction times

To find how many times faster sodium reacts, divide the lithium’s time by the sodium’s time: $\frac{84}{12} = 7$

So the sodium cube reacts $\boxed{7}$ times faster than the lithium cube.

Part (c)(ii): Predicting potassium’s reactivity

Potassium is below sodium in Group I. Since reactivity increases going down Group I (lithium reacts the slowest, sodium reacts faster, as shown by the shorter reaction time), a same-sized cube of potassium is expected to react faster than the sodium cube. Potassium is the most reactive of the three metals.

Final answers

(a) Density of lithium $= \boxed{0.53}\ \text{g/cm}^3$
(b) Density of sodium $= \boxed{0.97}\ \text{g/cm}^3$ ; density increases going down Group I.
(c)(i) Sodium reacts $\boxed{7}$ times faster than lithium.
(c)(ii) Potassium would react faster than sodium, because reactivity increases going down Group I.