Is radium sulphate soluble?

The reason you are expected to be able to predict the solubility of radium sulphate at AS is because there is a trend in solubilities of the group II sulphates in water. Magnesium sulphate is soluble, calcium sulphate is slightly soluble and strontium and barium sulphate are both pretty much insoluble. Therefore, you should predict that radium sulphate will be even more insoluble.