Can someone help me to understand this question?
2Na(s) + 2H2O (I) —> Na+(aq) + 2OH-(aq) + H2(g)

1.15 g of sodium react completely with an excess of water. The resulting solution is diluted to 0.250 dm3. Calculate the concentration of hydroxide ions in the dilute solution. Answer in mol / dm3 to two decimal places.